In my opinion, this isn't a strict rule.
It is a guideline because we want people who are responsible to be admins, because they are are example to others and they have to uphold the rules, and to do that they must themselves abide by the rules.
Also they need to be helpful generally, and so unless you prove yourself mature like TCP and Havok you won't get accepted. But to be honest, everyone has to prove themself. If you want to be admin people need to know you and so it is really not that you must be over 16, it is that you must be as mature as someone over 16, which is completely possible. That said however, some people over 16 are not mature enough either, so its really just a loose guideline.