Most men spend the best part of their lives making the remaining part wretched.

[La plupart des hommes emploient la meilleure partie de leur vie à rendre l’autre misérable.]

Jean de La Bruyère (1645-1696) French essayist, moralist

The Characters [Les Caractères], ch. 11 “Of Mankind [De l’Homme],” § 102 (11.102) (1688) [tr. Stewart (1970)]
    (Source)

(Source (French)). Alternate translations:

The greatest part of mankind employ their first years to make their last miserable.
[Bullord ed. (1696)]

The greatest part of Mankind imploy their first Years to make their last miserable.
[Curll ed. (1713)]

The greatest part of Mankind employ their first Years to make their last miserable. [Browne ed. (1752)]

Most men employ the first years of their life in making the last miserable.
[tr. Van Laun (1885)]

Most men make use of the first part of their life to render the last part miserable.
[Source]