‎One starts to get young

Youth has no age. 

Pablo Picasso

‎One starts to get young at sixty and then it's too late.
Pablo Picasso

She was good, quiet, dull, and amiable, and young only because she was twenty-three.
E.M Forster

Between thirty and forty, one is distracted by the Five Lusts;
Between seventy and eighty, one is prey to a hundred diseases.
But from fifty to sixty one is free from all ills;
Calm and still–the heart enjoys rest.
I have put behind me Love and Greed; I have done with Profit and Fame;
I am still short of illness and decay and far from decrepit age.
Strength of limb I still possess to seek the rivers and hills;
Still my heart has spirit enough to listen to flutes and strings.
At leisure I open new wine and taste several cups;
Drunken I recall old poems and sing a whole volume.
Meng-te has asked for a poem and herewith I exhort him
Not to complain of three-score, "the time of obedient ears."
Po Chu-i or Bai Juyi, translated by Arthur Waley

(The Buddha said men have five lusts: food, sleep, sex, money, fame. I presume this is what the poet means, though elsewhere I have seen a Chinese poet include love of flowers in the list, which seems odd.)

As the years increase, it always becomes easier to say 'Dare to be wise — sapere aude'. And after sixty, the inclination to be alone grows into a kind of real, natural instinct; for at that age everything combines in favor of it. The strongest impulse—­the love of woman’s society—­has little or no effect; it is the sexless condition of old age which lays the foundation of a certain self-sufficiency, and that gradually absorbs all desire for others’ company. A thousand illusions and follies are overcome; the active years of life are in most cases gone; a man has no more expectations or plans or intentions. The generation to which he belonged has passed away, and a new race has sprung up which looks upon him as essentially outside its sphere of activity.
And then the years pass more quickly as we become older, and we want to devote our remaining time to the intellectual rather than to the practical side of life. For, provided that the mind retains its faculties, the amount of knowledge and experience we have acquired, together with the facility we have gained in the use of our powers, makes it then more than ever easy and interesting to us to pursue the study of any subject. A thousand things become clear which were formerly enveloped in obscurity, and results are obtained which give a feeling of difficulties overcome.
Arthur Schopenhauer

Forty is the old age of youth; fifty the youth of old age. 

Victor Hugo