When did New York get so old? Le Corbusier once called the city “a geyser whose fountains leap and gush in continual renewal”, youthfully vigorous, of the Zeitgeist, while Old Europe slumbered. These days, though, the fountains trickle; cranes don’t swing over Manhattan quite like they used to; every time I visit now I notice how wrinkly the dame is looking these days, the marks of age all the more shocking beside the ebullience of its youth. The city that 70 years ago was the epitome of Modernism had its crown stolen by Los Angeles in the 1950s. These days, if you want fast-paced urbanism head to Shanghai, not Fifth Avenue.

Manhattan has become history, permanently frozen as it wants to remember itself, in a Buddy-can-you-spare-a-dime, browny-grey Gotham pallor of neon and Art Deco. That’s just how urban history goes. It had its day, between Edith Wharton and the Son of Sam. Now, in its dotage, New York has simply joined the ranks of all the other former greatest cities of the world – Athens, Rome, London, Vienna, Paris, et al – envying those racy whippersnappers in the Far East and occasionally indulging in dodgy midlife makeovers.

No bad thing, this. As recounted by Landmarks of New York, an exhibition about Manhattan’s heritage opening at the Royal Institute of British Architects next week, many fine buildings were lost before the city that never sleeps started taking afternoon naps. Even as Manhattan was preparing for its golden age, the New York Mirror was lamenting the loss in 1831 of New Amsterdam’s old Dutch houses through what Walt Whitman referred to as the city’s “pull-down-and-build-over-again” spirit. America’s heritage movement began, as Britain’s did, in the late 19th century, after the Civil War, when America became suddenly aware of its age, of the importance of writing history – with stone and brick as well as with words.

The British movement, though, was given fuel by the “creative destruction” of its Victorian cities’ heydays – when society is in tumult, you appreciate the physical markers of its history all the more – a fever that took hold in New York only when it entered its own heyday with the turn-of-the-century skyscraper. It hit its zenith only after 1945, when Manhattan went on a building spree, uprooting its past for sledgehammer “urban renewal”.

“It was then that this modern city truly awoke to its past, became aware that it had a history,” says the fabulously named Barbaralee Diamonstein-Spielvogel, curator of the exhibition, and doyenne of Manhattan’s preservationist scene for 40 years. “Before then for most New Yorkers, it had been just go, go, go, don’t look back.” Diamonstein-Spielvogel is chair of the Historic Landmarks Preservation Centre in New York and, under four mayors, was the longest-serving commissioner for the New York City Landmarks Preservation Commission, the body that designates, or “lists” historic buildings, a body born during the 1960s. This was a traumatic time, she recalls. Parts of the city had already awoken to the negative side of modernisation during the 1940s when Robert Moses, the city’s infamous planner, bludgeoned freeways studded with “housing projects” with the very Corbusian aim of unfurring the city’s arteries for the greater good – in other words, getting the middle classes to the ’burbs.

However, these freeways sped chiefly through poorer neighbourhoods – for political expediency. It was only when “creative destruction” reached centre stage that New Yorkers started thinking that preservationists were more than just pains-in-the-butt getting in the way of a good business deal.

In 1963, the vehement opposition to the destruction of McKim, Mead and White’s crystal-palace Penn Station to make way for the odious Madison Square Gardens spurred the creation of the commission, the city’s first organisation with the legal framework to protect buildings.

The philosophical wind was already changing within architecture and planning as within culture as a whole, away from Modernism’s broad-brush, towards the “PostModern” individualism of books such as Jane Jacobs’s The Death and Life of Great American Cities(1961), a preservationist’s gentle call to arms.

The era of preservation had begun, aided by the city’s economic collapse in the 1970s. Nobody wanted to build anything. New York? They couldn’t give it away. “People weren’t paying any attention,” Diamonstein-Spielvogel says. “So we just went ahead with designations.” They were both small – a colonial-style, 18th-century survivor in Queens, say – and iconic – such as the Chrysler Building (designated 1978) – together creating an image of “New Yorkness” at both neighbourhood and city-wide scales. This was also the moment when preservation got hip, by association. The city may financially have been on its uppers, but culturally it hadn’t been as vibrant for two decades.

The era of punk, disco and hip-hop had, as its architectural face, the downtown loft – old “cast-iron” warehouses discarded by the city’s shrivelled manufacturing sector, but recolonised by artists searching in Gotham’s past for that most p o s t -modern of qualities, a sense of p l a c e .

Manhattan ’ s cultural swansong was celebrated by the architect Rem Koolhaas in his book Delirious New York (1978), a love letter to a city in which Modernism and PostModernism were colliding in a “culture of congestion” of thrilling unpredictability.

When money started flowing through the city’s veins again in the 1980s, the philosophical shift was complete. Age, not modernity, was aspiration’s endgame. The rise and later dominance of gentrification in the city that gave birth to it meant that preservation was no longer an obstruction to the free market – it generated it. There may be some developers who still regard a landmark preservation order on a tasty building as economic suicide. “Most these days, though, see it brings with it a certain cachet,” Diamonstein-Spielvogel notes. “Buildings and neighbourhoods aspire towards inclusion,” while it remains the commission’s duty to sort the wheat from the chaff, like an architectural bouncer. The flip side, though, was that for two decades the city was in what Frank Gehry calls its “20year torpor. For new architecture, who looked to Manhattan?” The city got fat, conservative and complacent, awakening only, ironically, with 9/11, and then the arrival of Michael Bloomberg as mayor. He was wise enough to note that when a city falls in love with its own image, it dies.

Young blood – in architecture as much as economics – keeps a city alive. The list of the architects now working in Manhattan – Foster at Ground Zero; Rogers there and on the mammoth Javits Convention Centre and a huge new Downtown waterfront park; Jean Nouvel; Renzo Piano on the stately new New York Times skyscraper – should have Ken Livingstone green with envy.

Yet, equally, this year the Commission has totted up its highest number of preserved buildings for 20 years. Perhaps the loss of the twin towers woke New Yorkers to the fragility of their city. “Maybe now we have some balance again,” Diamonstein-Spielvogel says. “Preservation should never be dull, deathly.

Instead I think we’ve become a mature city. Older, wiser, better looking than ever before.” Landscapes of New York, Sept 6-Oct 3, RIBA, 66 Portland Place, W1 (020-7580 5533; www.architecture.com)

Monumental Manhattan: five of the city’s finest landmarks

St Paul’s Chapel and Churchyard Attributed to Thomas McBean and James Crommelin Lawrence, 1764-94. A reminder of when New York still looked to Europe – in this case St Martin’s-in-the-Fields, London – for inspiration.

Central Park Frederick Law Olmsted and Calvert Vaux, 1857. Marks the moment when Manhattan started to think of itself as beautiful and world class.

Brooklyn Bridge John A. Roebling and Washington and Emily Roebling, 1867-83. Took more than 20 lives to build what was for more than 20 years after the world’s longest suspension bridge. Walk across it to admire the stunning Manhattan skyline.

The Rockefeller Centre ( left) Hood, Godley & Fouilhoux, Corbett, Harrison & MacMurray, Reinhard & Hofmeister and Carson & Lundin, 1931-47. Built on John D. Rockefeller’s riches as a New Deal project to soak up the jobless.

Guggenheim Museum Frank Lloyd Wright, 1956-59. The perfect foil to the Mondrian-esque grid of Manhattan.

RIBA’s new president promises change

Sunand Prasad makes history on Saturday when he becomes the first ethnic minority president of the Royal Institute of British Architects in its 170-year history, after an election in which race screeched to the foreground when one of his rivals was revealed to be a member of the British National Party.

It doesn’t faze this Indian-born former chairman of the Society of Black Architects. Indeed, his manifesto barely mentions the issue. “It’s not a priority, no,” he says. “But I’ve always supported action on diversity – I’ve chaired two of the profession’s major studies on it.” Last autumn, though, when Richard Rogers proposed positive discrimination at the memorial lecture for the Stephen Lawrence Charitable Trust, Prasad described his stance as “patronising”.

“That isn’t to say race isn’t important,” he adds (only 2 per cent of British architects come from ethnic minorities). “But positive discrimination only backfires. If anything, the intake at architecture schools is more diverse than society. The problem begins after college, with drop-out, poor advancement. In any case, I believe class is the more pressing issue. When class collides with race, that can have a very corrosive impact. If anything, I’m better placed simply as a role model. That was part of why I stood.”

Instead, it is architecture’s wider stick-in-the-mud approach that really gripes him. Prasad was elected on a mandate of “change” – nebulous, until he points out that he means change everywhere, in all aspects of the profession.

“Where to start? Climate change? PFI? Getting young people active in architecture? The profession has been wrongfooted, caught napping by major changes in the political economy in the past ten years. It didn’t have a seat when all the major decisions were being made,” he says.

The design of our hospitals and schools has suffered. Prasad, though, is knee-deep in politics. His own firm, Penoyre & Prasad, designs healthcare buildings and schools. For years he worked for the Government’s Commission for Architecture and the Built Environment which, he says, “stole RIBA’s thunder” with initiatives such as his design quality indicators, architecture’s equivalent of the Stern Review that demonstrated to government number-crunchers that early investment in design could save money in the long term.

Prasad promises a nimbler, “more present” RIBA during his presidency. “Though it’s a long, slow process, like a big bendy supertanker. Half of it’s pointing the right way. The rest? Years behind.”

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