How did it become that everything is legal at 18?

Question

Like how did it become that you turn adult when you're 18? Who was the first to come up with this figure? Napoleon? Because we're I come from French civil law is used as a base.

Answer 1

It's based on old ROMAN CIVIL LAW.. and it's been around for longer than the country of France.  The Parisi adopted it AFTER they were conquered by the Romans