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.
It's hard at times to be the mature one when they're calling you things out of anger and it's hard not say something back. But I've done this many many times trying to understand why they're upset..but the thing was, they didn't understand me and went ahead and made assumptions instead :/