It's just a stereotype. Some people can't get over the fact that you're a teenage mother, they say things like "you lack mental maturity until 25!" or "teenage pregnancy costs taxpayers 11 billion each year!". They can't get over the fact that we're individuals who had sex, and that it can happen to anyone who does. They think that it's easy to group us all together, despite the fact that we come from all sorts of backgrounds, families, races, and social classes.
Don't listen to them. Use that negative energy as encouragement to prove them wrong. I too became a mother at fifteen, and I just recently finished high school and will begin college this fall. I used to be a disaster prior to getting pregnant because I was depressed and suicidal. My daughter is my strength and motivation to pursue post-secondary education and attain my dream career. She's not an obstacle, quite the contrary, without her I don't know where I would be today, probably dead.
My opinion on teenage mothers is no different than my opinion of older mothers. I can't tell if they were raped or not, I can't tell if they're still with their child's father or just escaped an abusive relationship, I can't tell if their parents are helping them, if they have the income for their children, or if they are still in school. I just see a girl with a baby, and I only hope the best for her. No judgement, if I have the opportunity to talk to her individually then I'll offer advice on breastfeeding, juggling homework and an infant, childcare, relationships, or anything that she's questioning. Sometimes, a teenage mother just needs a friend, since so many people hate us.