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.
There is nothing wrong with not wanting to have kids. They suck you dry emotionally, financially, mentally, and physically. You want to have 80 kids? Cool. But don't judge me because I don't want to waste my time with them.