• In England and other European league. Player can start playing in top league when he or she is 17+.

If the person live with parents or guardian. Otherwise,uefa will sanction the involving club for child abuse. Because UEFA care much of the players welfare and safety. As a reputable organization they don’t want to do involve in any act that promotes child abuse or child trafficking or labour.
But if he or she is not living with parents, he or she must wait for 18 years before signing first contract.
UEFA is flowing on any thing related to child labor or child trafficking or abuse.
18 years is universally accepted as maturity age.