The '60s. Maybe you had to be there.

Not that upwards of half a million people, hellbent on nostalgia, didn't try their best Saturday night to exhume the decade. They jammed New York's Central Park and sat peacefully on the Great Lawn for a free concert. They shared food and wine and passed around joints. They strummed guitars and hummed Crosby, Stills & Nash. They flashed the peace sign in front of TV cameras. For two dollars, some guy would paint your face.

And they witnessed the historical reunion of Paul Simon and Art Garfunkel, still crazy after 11 years of doing their own things. As revivalists, Simon and Garfunkel were the perfect choice. Their music has such a wistful, lullaby sound to it -- wind chimes, a distant train, the gentle tinkle of teeth falling into a glass -- even in '64 it sounded nostalgic.

It's just that so many of their fans, particularly the ones who came early to get close to the stage, were so young. Martha Rojas of Woodbridge, N.J., for instance, was 21.

"I'm very much into the '60s," she said. "I was probably born 20 years too late. I think the music of the '60s had more meaning than now. I like protest music."

Her friends Steve Cohen, 20, and Bill Oserin, 21, snickered affectionately as if they'd heard this rap before.

"I do," she insisted. "If you look into Simon and Garfunkel's lyrics, there's a lot of double meaning stuff. Like, 'the words of the prophets are written on the subway walls.' They're talking about graffiti, a way of expressing your opinions. I believe a lot of hippies were prophets."

A few feet away, Ian and Elizabeth Alterman, 23-year-old newlyweds, sat nestled around a sleeping bag encased in plastic. They'd been there since 11 Friday night. It was a spur-of-the-moment thing: They heard about the concert, threw some stuff together, including food and medicine (they both had colds), walked to the park from their apartment and slept in the rain.

"My parents turned me on to Simon and Garfunkel," said Elizabeth Alterman. "My father's a minister, and he turned me on to 'Bridge Over Troubled Water.' He wanted to use it in a sermon."

Ian Alterman first heard their music when he was 6 or 7; he's been a '60s music fan ever since. "Simon and Garfunkel and Crosby, Stills Nash & Young are the two biggest groups I remember. Today music has lost some of its innocence. It's come to the point where the package is as important as the music."

Despite their colds, the couple said they were having a great time and the crowd was very mellow. When they arrived, only 100 people were camped out. Thirty-five of them had guitars.

Vicki Wachtel, a 29-year-old bank assistant from Queens, agreed about the crowd. "I was starting to lose my faith in kids," she admitted. "I saw a violent trend there for a while. But this event has restored my faith in America's youth."

One older face in the crowd was Sylvia Seidman, a Manhattan English teacher with curly blond hair and a tight red sweater. She said she was over 50 and basically there for research purposes. "I have to know about this stuff," she explained, "to spark my kids, cuz I'm just a dirty old lady."

Seidman had a good spot for the show because her friend, Delores Bittman, about her age, had arrived at 11 a.m. and commandeered it. Meanwhile Seidman had gone to another part of the park and commandeered a tennis court for Bittman. The women played four hours of tennis, jogged two miles then returned for the concert. "You gotta keep in shape," said Seidman, lifting up her sweater and proudly flashing a bright red Leggs Mini Marathon T-shirt.

Seidman said she was overwhelmed by the friendliness of the young people around her. "I don't know whether its the pot or what, but I've never known such camaraderie. If more older people went to these concerts, there'd be no wars."

The concert itself couldn't have been more pleasing to fans of the '60s, both newcomers and old-timers. The clouds broke open an hour before the show. Simon and Garfunkel sang most of their old hits -- "Mrs. Robinson," "Homeward Bound," "America," "Scarborough Fair," "Sounds of Silence," "Feelin' Groovy," and "The Boxer" -- among others.

In fact, there was only one new song, a solo by Simon, called "The Late Great Johnny Ace." It was a quiet, moody piece that somehow tied in music and death, JFK and John Lennon. Unfortunately, it was disrupted when a freak jumped on the stage, lunged for Simon and had to be carried off.

But perhaps the evening's most authentically cosmic moment occurred just after the concert's finish, when searchlights roaming the scene suddenly focused on several hundred fans perched on an old fairy-tale castle across a lake behind the stage. No one had known they were there. In the darkness they had scaled a formidable wire fence and invaded the castle.

From their vantage point, they couldn't see the stage. But they could hear the music, and they could see the whole picture, the whole message, you dig? They could see 400,000 people dancing and singing and bathed in lights of gold. And they could see Simon and Garfunkel leaving backstage and being whisked off in their limos.