In short, while you have Strength, use it; when it leaves you, no more repine for the want of it, than you did when Lads, that your Childhood was past; or at the Years of Manhood, that you were no longer Boys. The Stages of Life are fixed; Nature is the same in all, and goes on in a plain and steady Course: Every Part of Life, like the Year, has its peculiar Season: As Children are by Nature weak, Youth is rash and bold; staid Manhood more solid and grave; and so Old-Age in its Maturity, has something natural to itself, that ought particularly to recommend it.

[Denique isto bono utare, dum adsit, cum absit, ne requiras: nisi forte adulescentes pueritiam, paulum aetate progressi adulescentiam debent requirere. cursus est certus aetatis et una via naturae eaque simplex, suaque cuique parti aetatis tempestivitas est data, ut et infirmitas puerorum et ferocitas iuvenum et gravitas iam constantis aetatis et senectutis maturitas naturale quiddam habet, quod suo tempore percipi debeat.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Cato Maior de Senectute [Discourse on Old Age], ch. 10 / sec. 33 (44 BC) [tr. Logan (1734)]
    (Source)

Original Latin. Alt. trans.:

To conclude, use that strength which you have while you have it; but when it is gone, require it not, unlesse you thinke it a seemly thing of young men, to require their child-hood againe, and ancient men their youth; There is but one course of age, and one way of nature, and the same simple, and to every part of age its own timelines is given; for as infirmity belongs to child-hood, fiercenesse to youth, and gravity to age, so the true ripe∣nesse of age hath a certaine natural gravity in it, which ought to be used in it own time.
[tr. Austin (17th C)]

In a word, my friends, make a good use of your youthful vigour so long as it remains; but never let it cost you a sign when age shall have withdrawn it from you; as reasonably indeed might youth regret the loss of infancy, or mahood the extinction of youth. Nature conducts us, by a regular and insensible progression through the different seasons of human life; to each of which she has annexed its proper and distinguishing characteristic. As imbecility is the attribute of infancy, ardour of youth, and gravity of manhood; so declining age has its essential properties, which gradually disclose themselves as years increase.
[tr. Melmoth (1820)]

In fine, I would have you use strength of body while you have it: when it fails, I would not have you complain of its loss, unless you think it fitting for young men to regret their boyhood, or for those who have passed on a little farther in life to want their youth back again. Life has its fixed course, and nature one unvarying way; each age has assigned to it what best suits it, so that the fickleness of boyhood, the sanguine temper of youth, the soberness of riper years, and the maturity of old age, equally have something in harmony with nature, which ought to be made availing in its season.
[tr. Peabody (1884)]

In fine, enjoy that blessing when you have it; when it is gone, don't wish it back -- unless we are to think that young men should wish their childhood back, and those somewhat older their youth! The course of life is fixed, and nature admits of its being run but in one way, and only once; and to each part of our life there is something specially seasonable; so that the feebleness of children, as well as the high spirit of youth, the soberness of maturer years, and the ripe wisdom of old age -- all have a certain natural advantage which should be secured in its proper season.
[tr. Shuckburgh (1895)]

Use then the gifts you have:
When gone, regret them not: unless as men
You are to ask for boyhood to return,
When older ask for you: there still must be
A certain lapse of years; one only way
Nature pursues, and that a simple one:
To each is given what is fit for him.
The boy is weak: youth is more full of fire:
Increasing years have more of soberness:
And as in age there is a ripeness too.
Each should be garnered at its proper time,
And made the most of.
[tr. Allison (1916)]

In short, enjoy the blessing of strength while you have it and do not bewail it when it is gone, unless, forsooth, you believe that youth must lament the loss of infancy, or early manhood the passing of youth. Life's race-course is fixed; Nature has only a single path and that path is run but once, and to each stage of existence has been allotted its own appropriate quality; so that the weakness of childhood, the impetuosity of youth, the seriousness of middle life, the maturity of old age -- each bears some of Nature's fruit, which must be garnered in its own season.
[tr. Falconer (1923)]

In short, enjoy the blessing of bodily strength while you have it, but don't mourn when it passes away, any more than a young man should lament the end of boyhood, or a mature man the passing of youth. The course of life cannot change. Nature has but a single path and you travel it only once. Each stage of life has its own appropriate qualities -- weakness in childhood, boldness in youth, seriousness in middle age, and maturity in old age. These are fruits that must be harvested in due season.
[tr. Freeman (2016)]