Hi Haydz of NZ,
I think it is a good thing to want to grow up, don't listen to those ppl!
What they're probably saying is you don't want to be too 'mature' that you forget to have fun.
But maturity is what God has called us too.
There're many kinds of maturity. Spiritual, emotional, mental, physical.
For spiritual maturity, spending time with God and His Word is the only way. Secondly, you would have to pick your Christian friends wisely too.
For the rest, I would say... the first step towards maturity is knowing yourself. Besides knowing who you are in God, do you know WHO you are that God made? Try taking a personality test, the MBTI is good.
When you honestly know who you are, I think the mature thing is to be responsible for the gifts and talents that God has blessed you with. TO spend your time developing them for God's glory. And another thing which comes in handy, is learning to prioritise. And if necessary, make sacrifices in order to make dreams come true.
Hope this was helpful