We are of opinion, then, that Youth is completed in the forty-fifth year.
And as Adolescence is in the twenty-five years which proceed mounting upwards to Youth: so the descent, that is, Old Age, is an equal amount of time which succeeds to Youth; and thus Old Age terminates in the seventieth year.
But because Adolescence does not begin at the beginning of life--taking it in the way which has been said--but about eight months from birth; and because our life strives to ascend, and curbs itself in the descent; because the natural heat is lessened and can do little, and the moist humour is increased, not in quantity, but in quality, so that it is less able to evaporate and be consumed; it happens that beyond Old Age there remains of our life an amount, perhaps, of about ten years, a little more or a little less; and this time of life is termed Extreme Old Age, or Senility.

Wherefore we know of Plato (of whom one may well say that he was a son of Nature, both because of his perfection and because of his countenance, which caused Socrates to love him when first he saw him), that he lived eighty and one years, according to the testimony of Tullius in that book On Old Age.

And I believe that if Christ had not been crucified, and if He might have lived the length of time which His life according to nature could have passed over, at eighty and one years He would have been transformed from the mortal body into the eternal.
Truly, as has been said above, these ages may be longer or shorter according to our complexion or temper and our constitution or composition; but, as they are, it seems to me that I observe this proportion in all men, as has been said, that is to say, that in such men the ages may be made longer or shorter according to the integrity of the whole term of the natural life.
Throughout all these ages this Nobility of which we speak manifests its effects in different ways in the ennobled Soul; and it is that which this part of the Song, concerning which we write at present, intends to demonstrate.

Where it is to be known that our good and upright nature makes forward progress in us in the reasoning powers, as we see the nature of the plants make forward progress; and therefore it is that different manners and different deportment are to be held reasonable at one age rather than at another.

The ennobled Soul proceeds in due order along a single path, employing each of its powers in its time and season, or even as they are all ordained to the final production of the perfect fruit.