Often an opinion, a custom, seems absurd when we are young, and advancing in life, we find the reason. Mustn’t we conclude that certain habits aren’t so ridiculous? One is lead to think sometimes that they were established by people who had read the entire book of life, and that they are judged by people who, despite their esprit, have only read a few pages.

[Souvent une opinion, une coutume commence à paraître absurde dans la première jeunesse, et en avançant dans la vie, on en trouve la raison; elle paraît moins absurde. En faudrait-il conclure que de certaines coutumes sont moins ridicules? On serait porté à penser quelquefois qu’elles ont été établies par des gens qui avaient lu le livre entier de la vie, et qu’elles sont jugées par des gens qui, malgré leur esprit, n’en ont lu que quelques pages.]

Nicolas Chamfort (1741-1794) French writer, epigrammist (b. Nicolas-Sébastien Roch)

Products of Perfected Civilization [Produits de la Civilisation Perfectionée], Part 1 “Maxims and Thoughts [Maximes et Pensées],” ch. 1, ¶ 21 (1795) [tr. Siniscalchi (1994)]
    (Source)

Compare to also Chamfort.(Source (French)). Alternate translations:

Often in early youth an opinion or custom seems absurd to us, which, with advancing years, we discover has some justification and so appears less absurd. Ought we to conclude from this that certain customs are not so ridiculous as others? One might sometimes be tempted to think that they were established by people who had read the book of life through, and that they are judged by those who, despite their intelligence, have only glanced at a few pages.
[tr. Hutchinson (1902)]

Often an opinion or custom seems absurd to us in early youth; but as we advance in life we see the reason for it, and it appears less fatuous. Must we conclude from this that certain customs actually are less absurd? One is sometimes led to suppose that they have been established by folk who have read the whole of the book of life, and that they are criticized by those who, in spite of their intelligence, have only read a page or two at best.
[tr. Mathers (1926)]

Often an opinion, a custom, seems absurd to begin with, when one is very young, and as one advances in life one learns the reason for it, and it seems less so. Must one conclude, then, that certain customs have become less ridiculous? At times one is drawn to the conclusion that they were established by persons who had read the book of life entire, and are judged by others who have read only a few pages.
[tr. Merwin (1969)]

Often an opinion, or a custom, begins to seem absurd in one's early youth, and, as life advances, one finds the reason for it; it seems less absurd. Is one ot conclude that certain customs are less ridiculous? One would sometimes be inclined to think that they had been laid down by people who had read the entire book of life, and that they are judged by people who, in spite of their intellect, have only read a few pages of it.
[tr. Pearson (1973)]

To the very young some opinions or customs seem absurd, but as you grow older you realize the reason behind them and they seem less absurd. Are we to conclude that certain customs aren't as ridiculous as they seem? There are times when you feel that they've been created by people who've read the book of life from cover to cover and that they're being judged by those who, however intelligent, have read only a few pages.
[tr. Parmée (2003), ¶ 20]