Samuel Butler in The Way of The World summed it up very well.
Some satirists have complained of life inasmuch as all the pleasures belong to the fore part of it and we must see them dwindle till we are left, it may be, with the miseries of a decrepit old age.I am not young enough to know everything.
To me it seems that youth is like spring, an overpraised season-- delightful if it happen to be a favoured one, but in practice very rarely favoured and more remarkable, as a general rule, for biting east winds than genial breezes. Autumn is the mellower season, and what we lose in flowers we more than gain in fruits. Fontenelle at the age of ninety, being asked what was the happiest time of his life, said he did not know that he had ever been much happier than he then was, but that perhaps his best years had been those when he was between fifty-five and seventy-five, and Dr Johnson placed the pleasures of old age far higher than those of youth. True, in old age we live under the shadow of Death, which, like a sword of Damocles, may descend at any moment, but we have so long found life to be an affair of being rather frightened than hurt that we have become like the people who live under Vesuvius, and chance it without much misgiving.
J. M. Barrie
The London show's master of ceremonies noted that "he's fifty-two now but so fears old age that he prefers to think of himself as eleven centigrade."
Jeremy Bernstein, Cranks, Quarks, and the Cosmos (1993)
"Having Fun with Tom Lehrer"
The older you get, the more important it is not to act your age.
You can't help getting older, but you don't have to get old.
When you're young, you don't know, but you don't know you don't know, so you take some chances. In your twenties and thirties you don't know, and you know you don't know, and that tends to freeze you; less risk taking. In your forties you know, but you don't know you know, so you may still be a little tentative. But then, as you pass fifty, if you've been paying attention, you know, and you know you know. Time for some fun.
George Carlin, Brain Droppings (1997)
Old Boys have their Playthings as well as young Ones; the Difference is only in the price.
A person's maturity consists in having found again the seriousness one had as a child, at play.
Friedrich Wilhelm Nietzsche, Beyond Good and Evil (1885-1886)
It takes a very long time to become young.
Wrinkles should merely indicate where smiles have been.
Mark Twain, Following the Equator (1897)
"Pudd'nhead Wilson's New Calendar"
You're never too old to become younger.