Basically, your problem can be separated into two subproblems:
- You have an age limit that seems to not work in all situations;
- You have a lot of immature users that cannot be banned immediately for not meeting the age requirement because it is not known whether they meet the age requirement.
First, let me say something you may not like: you are likely not going to solve any of those problems without making your site's rules, policies and/or registration procedures more complicated.
Second, let's look at the age limit problem. Of course, if you are legally required to have this age limit, there is no option of simply removing it. Also note that even if the age limit can be modified, it may not be a good idea to do so for the following reasons:
- If you reduce (or, which seems worse, remove) the age limit, the community, given the immaturity problem, will probably disapprove that. In addition, some users that used to be underage but will no longer be underage may register again or ask to have their bans removed. (The latter can be resolved, for instance, by adding a clause like "All users banned for not meeting the previous age limit may not have their bans removed before they meet the previous age limit. All new users must only meet the new age limit."). For these reasons, making the age requirement more lax (or worse, removing it altogether) is nearly definitely a bad idea.
- If you increase the age limit, you may have the problem of users who no longer meet the age limit. You can of course specify that only new users have to meet the new age limit, but this may be viewed by some users as an overcomplication. Some users may approve of increasing the age limit (because, unfortunately, many consider age and maturity to be bound very closely and think that increasing the age limit increases maturity of the site's users).
Also attempting to validate your age limit by making registration more complicated may cause some theoretically constructive users to not register at all. Needless to say, this in not particularly good.
So I think that the best solution for the first subproblem is as follows: do not change your age limit and don't require users to validate it, but also enforce it whenever a user is underage.
Third, let's talk about user maturity. I think there is a rather simple solution you may not have considered, that is, adding a rule that demands users to behave in a mature manner. Assuming that your moderation team is competent, this is unlikely to cause incidents regarding misuse of this rule. Obviously, since it seems that the maturity problem is severe, you want to ensure that this rule is enforced in a very strict manner (e. g. by a progressive warning/ban system applied to a user at first signs of immature behaviour. I think that users who consider immaturity to be a severe problem are very likely to approve of that rule.
Short version of the answer: I consider the following course of action to be optimal:
- Keep your age limit, but don't validate it;
- Add a rule requiring that users behave in a mature manner, and be strict when enforcing the rule.
Personal experience: I have more than 2 years of experience being an administrator of a MediaWiki wiki. I have also been regularly visiting several websites with varying approaches to the age/maturity problem.
I am not a native speaker of English. Please correct mistakes in this answer, if any.