There are no laws in OP about anything. If Oda wants to make a 50 year old a rookie he'll make them.
Brook is 90 yo and still getting stronger lol.
Still, I am explaining how different culture see a guy.
That u are a man at 18, or 21 is mostly American point of view.
Many women around their 40-50-60is in Italy (my country) consider males very immature and not really a man until their 30is.
Japan is similar, that's why Shanks looks pretty young for being 25.
But honestly, I am very similar to him in this regard.