And, briefly, it is to be known that, as it has been said above in the sixth chapter of the third treatise, the Church makes use of the hours temporal in the division of the day, which hours are twelve in each day, long or short according to the amount of sunlight; and because the sixth hour, that is, the midday, is the most noble of the whole day, and has in it the most virtue, the Offices of the Church are approximated thereto in each side, that is, from the prime, and thence onwards as much as possible; and therefore the Office of prime, that is, the tertius, is said at the end of that part, and that of the third part and of the fourth is said at the beginning; and therefore, before the clock strikes in a division of the day, it is termed half-third or mid-tertius; or mid-nones, when in that division the clock has struck, and thus mid-vespers.
And, therefore, let each one know that the right and lawful nones ought always to strike or sound at the beginning of the seventh hour of the day, and let this suffice to the present digression.
Returning to the proposition, I say that Human Life is divided into four ages or stages. The first is called Adolescence, that is, the growth or increase of life; the second is called Youth, that is, the age which can give perfection, and for this reason one understands this Youth to be perfect, because no man can give except of that which he has; the third is called Old Age; the fourth is called Senility, Extreme Old Age, as has been said above.
Of the first no one doubts, but each wise man agrees that it lasts even to the twenty-fifth year; and up to that time our Soul waits for the increase and the embellishment of the body. While there are many and very great changes in the person, the rational part cannot possess perfectly the power of discretion; wherefore, the Civil Law wills that, previous to that age, a man cannot do certain things without a guardian of perfect age.
Of the second, which is the height of our life, the time is variously taken by many. But leaving that which philosophers and medical men write concerning it, and returning to the proper argument, we may say that, in most men in whom one can and ought to be guided by natural judgment, that age lasts for twenty years. And the reason which leads me to this conclusion is, that the height or supreme point of our arc or bow is in the thirty-fifth year; just so much as this age has of ascent, so much it ought to have of descent; and this ascent passes into descent, as it were, at the point, the centre, where one would hold the bow in the hand, at which place a slight flexion may be discerned. 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.