The course of a man’s life is certain. The path that we follow goes in only one direction. Every mile is distinctly marked with its own peculiar characteristic — the vulnerability of infants, the animal high spirits of adolescents, the seriousness of adults, the maturity of old men — and at each of these stages we must accept gracefully what Nature grants us.

[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
De Senectute [Cato Maior; On Old Age], ch. 10 / sec. 33 (10.33) (44 BC) [tr. Cobbold (2012)]
    (Source)

(Source (Latin)). Alternate translations:

The cours and the weye of age is certeyne and determyned by nature, whiche hathe onely awey which is symple & is nothyng different more in the one than in the othir. But each go by that symple and determyned wey aftir the degrees in their cours from the one age in to that other. And yet nature had given to every part of age his owne propre season and tyme, and hir pertynent cours of usage in kynde. That is to witt, that sekenesse and maladye is appropryd to the age of puerice in childhode, & cruelte is appropryd to the age of yongth, worshipfulnesse and sadnesse of maners be appropryd to the age of virilite whiche is the fyfthe age. Moderaunce and temperaunce be appropryd to olde age. Eueriche oweth to have sumwhat naturelly and appropryd unto that whiche may be gadird in his tyme.
[tr. Worcester/Worcester/Scrope (1481), Part 3]

The race and course of age is certain; and there is but one way of nature and the same simple; and to every part of a man's life and age are given his convenient times and proper tempestivities. For even as weakness and infirmity is incident to young children, lustiness and bravery to young men, and gravity when they come to ripe years; so, likewise the maturity or ripeness of old age have a certain special gift given and attributed to it by nature, which ought not to be neglected, but to be taken in his own time and season when it cometh.
[tr. Newton (1569)]

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 ripenesse of age hath a certaine natural gravity in it, which ought to be used in it own time.
[tr. Austin (1648)]

Simple, and certain Nature's wayes appear,
As she sets forth the seasons of the year.
So in all parts of life we find her truth,
Weakness to childhood, rashness to our youth:
To elder years to be discreet and grave,
Then to old age maturity she gave.
[tr. Denham (1669)]

Every Age has something in it, peculiar to it self: as Weakness to our Infancy, an unguided Warmth to Youth, Seriousness to Manhood, and a certain Maturity of Judgment to Old Age, which we may expect to reap the Fruits of, when advanced to it.
[tr. Hemming (1716)]

Life has a sure Course, and Nature but one Way, that that too simple and plain. And to every Part of Man's Age a peculiar Propriety of Temper is given: Thus Weakness in Children, a Boldness in Youth, and a Gravity in Manhood appears; and a full Ripeness of Years has always something which seems natural to it, and which ought to be made use of at a proper Time.
[tr. J. D. (1744)]

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.
[tr. Logan (1750)]

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 (1773)]

The course of life is fixed, and the path of nature is one, and that simple. And its own proper seasonableness has been given to each division of life; so that both the feebleness of boys and the proud spirit of young men, and the gravity of a now settle period of life, and the maturity of old age, has something natural to it, which ought to be gathered in its own season.
[Cornish Bros. ed. (1847)]

There is a definite career in life, and one way of nature, and that a simple one; and to every part ot life its own peculiar period has been assigned: so that both the feebleness of boys, and the high spirit of young men, and the steadiness of now fixed manhood, and the maturity of old age, have something natural, which ought to be enjoyed in their own time.
[tr. Edmonds (1874)]

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)]

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)]

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)]

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)]

The course of life is clear to see; nature has only one path, and it has no turnings. Each season of life has an advantage peculiarly its own; the innocence of children, the hot blood of youth, the gravity of the prime of life, and the mellowness of age all possess advantages that are theirs by nature, and that should be garnered each at its proper time.
[tr. Copley (1967)]

Life and nature have but one direction
Easy to take, without correction.
Each of life’s rite of passage dates
Has its own distinguishing traits:
A child’s weakness
A youth’s boldness
An adult’s authority
An old man’s maturity
And each with a certain natural zest
To be reaped when it’s time for its harvest.
[tr. Bozzi (2015)]

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)]