Does it matter if you're 15?
I mean, it's all that simple. Some people is very immature at the age of 18, where some is very mature.
Just be happy, this means at the age of ~18, you will be doing younger moms which boyfriend left them in the range of ~22-28, which is pretty hooooooooooot. And then they aint all immature (lol, no wonder eh?).
When you grow older, you'll realise how lucky you are, and moved on in the life, instead of being stuck in the childish part. Your friends will be older, since those on your age simply is too immature.
Trust me ;-).