I heard it plenty when Frank Sinatra was deep into his 70s, still selling out arenas around the world. Also when Cab Calloway was in his 80s, joyously strutting up and down the stage snarling Minnie the Moocher. Ditto septuagenarian jazz singer Ella Fitzgerald and nonagenarian jazz trumpeter Doc Cheatham.

And, of course, now regarding singer Tony Bennett, who will turn 93 in August.

The implication is that after a certain age, musicians no longer sound as they did in their "youth" or "prime". These are two loaded terms that, in this context, seem designed to diminish artists of exalted age and experience.

So why do these legends sustain careers for so long?

"I didn't try to do that, but it's happening," says Bennett.

He has few peers or precedents when it comes to pop/jazz singers flourishing in their 10th decade.

"So I'm thrilled about it. I've had a wonderful life doing what I love to do, and the public accepts me for it."

The turnouts Bennett draws around the world suggest he's right, yet more than one sceptic has said to me that the old master should have yielded the spotlight years ago. How wrong they are. When a performer of Bennett's vintage takes the stage, we're encountering more than just how they sound on any given night (though Bennett's command of a large repertoire, remarkable accuracy of pitch and inextinguishable sense of style defy the passage of time).

Because these legends have been in our lives for so long, their very appearance onstage calls forth memories richly worth treasuring. Better still, elder performers represent eras that without them simply would slip into the history books and, then, into obscurity.

So when Bennett, for example, sings Because of You or Rags to Riches, tunes he helped make famous, he's showing us how this music ought to sound.

Younger performers, of course, will invent their own worthy interpretations, but Bennett, Sinatra, Fitzgerald and other innovators set a standard against which the rest will be judged.

The ultimate example is Sinatra, whose decades-long career set an unassailably high bar for several musical traditions, from his dulcet-toned melody-making of the 1930s and '40s to his euphoric swing singing and brooding ballads of the '50s to his extraordinarily subtle bossa nova work of the '60s and so forth. When it comes to interpreting jazz-tinged repertory, Sinatra taught the world how to sing.

"He meant everything," says Bennett, who long considered Sinatra not only his best friend but a role model.

"Because when I was on my way up, he was way over on top. He was so supportive of me ... Whenever he said something good about me, it made me feel that I was communicating."

More than anything, Sinatra surely taught Bennett and all who have come after how to turn a mere pop song into a work of high art.

When performers of this stature are still able to perform in the autumn of the years (or even the winter), we are not merely entertained but enlightened, an opportunity not to be missed.