Grown-ups never understand anything by themselves, and it is tiresome for children to be always explaining things to them.

[Les grandes personnes ne comprennent jamais rien toutes seules, et c’est fatigant, pour les enfants, de toujours et toujours leur donner des explications.]

Antoine de Saint-Exupéry (1900-1944) French writer, aviator

Le Petit Prince [The Little Prince], ch. 1 (1943) [tr. Wood (1945)]
    (Source)

(Source (French)). Other translation:

Grown-ups never understand anything by themselves, and it is exhausting for children to have to provide explanations over and over again.
[tr. Howard (2000)]