Will you still need me, will you still feed me,

When I'm sixty-four?

At least Paul McCartney had the prescience to ask a question rather than making a statement. Other Sixties icons have had to become living disavowals of their statements of youthful arrogance. "Hope I die before I get old," wrote Pete Townshend, a chap who now drives around in a mobile home. "I'd rather be dead than singing 'Satisfaction' when I'm 45," sneered the butterfly Mick Jagger, who is now also eligible for a bus pass.

But time has no favourites. Tomorrow, just a month after he split up from his second wife, McCartney actually is 64, and the words of one of his best-known songs - if you can select for fame from such a stellar oeuvre - ring with unhappy irony.

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When I get older losing my hair,

Many years from now,

Will you still be sending me a Valentine

Birthday greetings bottle of wine?

If I'd been out till quarter to three

Would you lock the door?

He has not lost his hair, though he dyes it with a colour which many find a little too vibrant in its red-brown hue. And just at present there is no one at home to lock him out if he goes on a bender - nor anyone indoors to nag him if, in deference to his advanced years, if he decides just to stay home and light a spliff. But, as to the Valentines, I expect he will still get a few.

One of the most noticeable qualities of the public and press reaction to the news of his marriage break-up has been the ferocity of its protective affection for the former Beatle, which has found its obverse in the vituperative attacks on his estranged wife Heather Mills McCartney. But that tells us more about us than it does about her.

You'll be older too,

And if you say the word,

I could stay with you.

And so he has. For Paul McCartney's 64th birthday is not merely a personal event. It is a cultural milestone for a generation too. For, such is the nature of celebrity, McCartney is one of those people who throughout his life has, in some intangible way, represented the hopes and aspirations, joys and sorrows of those who were born in the baby-boom era which had its adolescent awakening in the Sixties and Seventies.

"When I'm Sixty-Four" was the first song the Beatles laid down on 8 December 1966, the day they began recording what Rolling Stone recently adjudged to be the most influential rock album of all time, placing it at the head of their 500 Greatest Albums listing. Sgt. Pepper's Lonely Hearts Club Band was, in the words of its producer George Martin, "the most innovative, imaginative, and trend-setting record of its time". The most influential high-brow of the day, Kenneth Tynan, went further, describing the album as "a decisive moment in the history of Western civilisation".

But McCartney's influence went well beyond being one of the keystones of the psychedelic rock revolution of the 1960s. He may have been going in and out of style since Sgt Pepper taught the band to play (almost 40 years ago today), but his overall career has made him the most successful pop composer of all time. According to Guinness World Records he has sold more records (400 million), penned more No 1 hits (50) and played to a bigger paying audience (350,000) than any other songwriter. It has made him the richest rock star in the world, with an estimated personal fortune of £825m. His song "Yesterday" is one the most recorded songs ever, with more than 3,000 versions. And yet, in a world all too often characterised by envy, McCartney's success, like his genius, is embraced by his generation almost as a wishful projection of their own life journey.

I could be handy, mending a fuse

When your lights have gone.

You can knit a sweater by the fireside

Sunday mornings go for a ride,

Doing the garden, digging the weeds,

Who could ask for more?

There are in that echoes of McCartney's happy childhood. He was born to working-class parent at Walton Hospital in Liverpool. His Irish Catholic mother, Mary, had worked as a nurse and midwife there. His British Protestant father, Jim, was a cotton salesman and a self-taught pianist and dance-band leader. "He had a lot of music in him, my dad," the singer has said. "He taught me and my brother harmony. Whenever John [Lennon] sang, I automatically sang in harmony with him, and that's due to my dad's teaching."

They pair had met when John Lennon's band, the Quarrymen, was playing at a local church fete. With the addition of McCartney, then aged 14, and George Harrison (who became lead guitarist because he knew more chords) the band morphed into the Silver Beetles and then the Beatles.

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They learned their craft playing long sessions in Hamburg, Germany, where the band first played a version of the song that would become "When I'm Sixty Four", as a bit of what Paul called acoustic "rooty-tooty" when the power failed and their amps went off. It lurked in the background throughout the Beatles' heyday in which they became the most extraordinary international hit-making phenomenon the pop world had seen, influencing everything from music to fashion and spirituality to politics.

The song embodied in its cod sentiments what Paul McCartney wanted for his own children. Not long after it was recorded, at the launch party for Sgt. Pepper he met the photographer Linda Eastman. They married and had three children.

For all his fame what Paul craved for his family was the happiness and security of his own childhood. He sought out ordinariness, sending his kids to the local comprehensive, eating Linda's home-cooked (and not always very appetising) food and even insisting that she did the family's laundry so that the house would smell of ironing to evoke the memories of his childhood home. Guests invited to lunch might be treated to a personal performance at the piano afterwards, but then they were expected to help to wash up.

There was an irony in the idyll. For though McCartney was usually considered the most conventional of the Beatles - an image which Lennon fostered to contrast with his own as a wayward genius - the image was misleading. McCartney was never quite what he seemed.

While the rest of the Beatles retreated to secure stockbroker belt estates McCartney lived in the centre of London exploring the more avant garde of the capital's influences. It was he, not Lennon, who cultivated the London art scene, launching the art shop in which Yoko Ono first met the Beatles. He became a friend of leading art dealers, explored experimental film, and attended "alternative" events, such as the launch party for International Times which he attended, as so often, in disguise. He was the first British pop star to admit openly to using LSD.

Yet Lennon was always accounted the radical. It was that, as part of the long years of friction and rivalry, along with what the other three Beatles saw as the baleful influence of Yoko Ono, that finally eroded the men's friendship.

Every summer we can rent a cottage,

In the Isle of Wight, if it's not too dear

We shall scrimp and save

Grandchildren on your knee

Vera, Chuck & Dave

In 1966, the year Sgt. Pepper was recorded, McCartney's father Jim reached the age of 64. His son decided to record the old tune as a tribute. At that point grandchildren were but a dream for McCartney Snr. When the real ones came along McCartney christened them Mary, Stella, James - two of them named after his parents.

There was a poignancy to that. For the happy old age the song celebrated was one which McCartney's parents did not enjoy. His mother, Mary, died of breast cancer when he was just 14. It shattered the happiness and security of Paul's life and it was more than a decade later that he seemed to become reconciled to it; the song "Let It Be" is based on a dream he had about her a decade after her death. It was a cruel blow to McCartney that the disease which killed his mother was then diagnosed in his wife Linda, who died three years later in 1998 after almost 30 years of marriage.

Send me a postcard, drop me a line,

Stating point of view

Indicate precisely what you mean to say

Yours sincerely, wasting away

Several lost years followed Linda's death, much as they had in 1970 after the Beatles broke up when McCartney was just 27. After long periods of introspection McCartney tried to reinvent himself. His post-Beatles solo career was chequered, with considerable commercial success but little critical acclaim for material such as "Mull of Kintyre", one of the best-selling British singles ever. He sought refuge in collaborations, with a succession of musicians: Stevie Wonder, Michael Jackson, Elvis Costello, Youth of Killing Joke, Super Furry Animals and the remixer Freelance Hellraiser.

After Linda's death he attempted a similar reinvention, remarrying another campaigning blonde, but though he and Heather Mills had a daughter, the marriage was short-lived.

Will you still need me, will you still feed me,

When I'm sixty-four?

It will be a birthday tinged with sadness that the woman he hoped would answer that question will offer no reply. But Paul McCartney can take some comfort that there are millions of fans out there who will still answer resoundingly in the affirmative.

A Life in Brief

BORN James Paul McCartney, Liverpool, 18 June 1942. Mother Mary, a nurse; father Jim, an amateur band leader.

FAMILY Married, 1969, Linda Eastman (died 1989), three children, Mary, Stella and James, plus one adopted daughter Heather (child of Linda's first marriage); 2002, Heather Mills, daughter Beatrice Milly born in October 2003 (separated 17 May 2006).

EDUCATION Art college, Liverpool.

CAREER Songwriter, singer and bass player with the Beatles (1958-1970), the biggest musical act of the 20th century. Add to that a multifarious solo career since then and McCartney is the most successful pop composer of all time. He was knighted in 1997.

HE SAYS "None of us wanted to be the bass player. In our minds he was the fat guy who always played at the back."

THEY SAY "Paul McCartney is a genius and the Beatles are the greatest band to ever walk this earth." - Ozzy Osbourne, singer