Not every who has kids at a young age "grows up very fast." My husband's parents were both 20 and not exactly mature when he was born, or when his brother was 2 years later. The parents divorced within a few years and neither of them really made the kids a priority (obliviously exposing them to everything from horror movies to abusive step-parents), and when the boys reached their teen years and their mother realized that they were going to require actual work and patience, she kicked them out to live with their grandmother.
A few years later, she and her current husband adopted two kids from a family member who couldn't take care of them, and when they got to be rebellious teens, off they went to live with
their grandmother. My husband and his brother turned out just fine thanks to their incredible grandparents; I can only hope for the same for the adopted kids.
At 22, I definitely feel the pangs of the maternal instinct (especially since a friend of ours recently had an adorable baby), but I know that I'm not ready to have kids for a few more years. I can see myself fucking up in some of the same ways that my MIL did, like wanting to spend time with my friends without realizing that my child needed company, or resenting my kids because I couldn't go out and party like I wanted to. For now the plan is to get a kitten or a puppy, finish school, and stay on birth control until I'm damn sure.