underage

An underage person is someone who hasn't yet reached the age at which they are considered grown up and responsible enough to make their own decisions.

When we talk about something being underage, it usually means that it's not allowed for people under a certain age to do it. For example, you might see warnings on movies or video games saying "Under 17 requires accompanying adult" – this is because the rules say that kids need a grown-up with them when they watch or play these things. It also shows up in laws and regulations about drinking, voting, driving, and other stuff where there's an age limit.