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.
A classy person is mature enough to forgive while a trashy person holds a grudge AND says untrue shit about you TO YOUR FACE. Never let someone talk untrue shit about you to your face, set them straight and tell them you feel sorry for them and hope they'll grow up, then leave.