meaning: “be young and shut up” is a term used in 1968 made by students of “L’école Supérieure des Beaux Arts de Paris” The man in black in this picture represent Charles De Gaulle, the kid in white is most likely a kid under the age of 21. The gesture represented here is that De Gaulle is refusing to give the liberty of expression to the youth of France back in 1968. This was one of the most famous slogans used at the time.
Other slogans I liked:
Les murs ont la parole (the walls have a saying)
Même si Dieu existait, il faudrait le supprimer (Even if god existed we need to delete him)
On achète ton bonheur. Vole-le (We buy your happiness, steal it)
Soyez réalistes, demandez l'impossible (Be realists, demand the impossible)
And the one you probably know: La Poésie est dans la rue (Poetry is in the streets)