It is not the young people that degenerate: they are not spoilt till those of maturer age are already sunk into corruption.

[Ce n’est point le peuple naissant qui dégénere ; il ne se perd que lorsque les hommes faits sont déja corrompus.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Spirit of Laws [The Spirit of the Laws; De l’esprit des lois], Book 4, ch. 5 (4.5) (1748) [tr. Nugent (1750)]
    (Source)

On society's need to educate the young into a love for the republic.

(Source (French)). Other translations:

It is not young people who degenerate; they are ruined only when grown men have already been corrupted.
[tr. Cohler/Miller/Stone (1989)]

It is not the rising people that degenerates ; it only declines when the fully-formed men are already corrupted.
[tr. Stewart (2018)]