How are some teens able to understand life?
Well they have there own view if life. it may seem right to one person and completely wrong to the next. So they do understand life to their own extent.
Talk to people who have already been where you at now, (The meaning of life has been an unanswered question for eons.) by doing so, you may learn the do's and don't's, and thus ,be able to compose your own understanding of life.
It depends on how they grew up and who they grew up with. Maturity is also a factor, some teenagers mature earlier than others. And experience can also help them.