>>Why is teen pregnancy bad?
>>Both my grandmothers started families in their teens and I never noticed anything particularly bad about them.

Lots of statistical reasons to avoid teen pregnancy in the contemporary First World. Social/financial effect on mum is self-explanatory, especially if it affects her educational prospects and dad skives off. 80% of such mums rely on welfare at some point. She's also at higher risk of some serious complications of birth. If dad sticks around there are higher alcohol and substance abuse risks for him as well, including lower educational status and reduced earning potential forever after. Kids are not spared: kids of teen mums suffer more health and social problems than those of older mums. They too exhibit lower educational achievement as well as more delinquency and higher risk of repeating the cycle themselves.