This game is international, and not limited solely to the US, making US founding documents irrelevant in this situation.
Were we to take this route, your parent/legal guardian (Should you live in the US) can make decisions for you, and your legal rights are forfeited to them until you become a legal adult (18).
Going into the game, people can chose to set restrictions or not when making a clan. Just as trophies are a restriction/requirement, age and maturity may be requirements as well. As a general rule, children are less mature than adults, which is why most clans allow maturity to prevail over age, and set maturity requirements instead of age requirements.
There is nothing wrong with age and/or maturity requirements- and let's face it. A clan that requires you to be an adult probably isn't having discussions that are relevant, interesting, or even appropriate for children.

