Maturity is more to do with the mind than with your age. Sure, your body physically matures. You cease to grow, you hit puberty. Mentally and physically women mature sooner than men, but mentally no one is mature when they turn 18.
I heard a man that I worked with once say that quite people were idiots & that they are quite because they can't think of anything intelligent to say. What was so funny to me was that I was quite & thought they were all idiots as I sat there day after day listening to them talk.