Dreizehnn
It is a good question but I think it is more from the experience of the person who reincarnates, for example if that person is young from 13 to 21 and has not experienced a hard life then his mentality would be very childish and stupid but if that person lived a hard life that forced him to mature and make decisions that ensure his survival, that's why I say that it depends on the past of the reincarnated
I don't think their level of maturity would increase that much? It would of course depend on the environment he would be brought up in for example if they have harsh childhood, then their previous experience can help them avoid a lot of traumas and pain. In such situation I imagine they would be a lot more mature is some ways but still basically behaving as teenagers additionally because they've been stuck in this phase of growing up they would probably have hard time changing their ways to become true adults.