Im a 50+ and often hear a saying "If your not mature now why start trying?" Seriously though, as an ex teacher I have come across many who in their early years would be classed as mature amongst their peers. Someone can be mature for their age at 7 or 10 or 16. That is not quite the same as being a mature adult - and believe me I know a lot of adults who are not mature. There are some younger ones I can have a better conversation with than older ones, BUT.. and that is a big BUT - putting foul language to one side (which in my opinion is just that) - there are many topics which the younger generation cannot relate to due to lack of life experience. This does not make them immature - just lacking in the experience to understand why a given situation will produce a given response.
I have to agree with keeping the age limit enforcement in. There are many who will, as has been suggested elsewhere, lie about their age to play on certain servers. That is not a mature response - for some it is curiosity - for others it is a down right dangerous attitude to take with all those supposedly "mature" adults about.
My grand children have been playing minecraft for a long time and they are both less than 10. To solve their desire to play on a server with others I built a server just for the family to access. Now they happily play with the people they know and can trust. And yes occasionally they invite a friend to play - but only after Ive spoken with the child's parents.