Adults and minors opinions?

why does everyone automatically assume if you are a teenager you are immature and childish? i have been on this site reading people's stories and yes it is sad when a married adult couple want a child and they can't have one but yet every single teenager it seems like gets pregnant. but i want to know from everyone...if you are a teenager why are you automatically assumes as childlike, immature, irresponsible, and cannot support another life?