I define personal responsibility by taking responsibility for your actions and accepting the consequences that come with them. I also define it by people accepting the fact that they are solely responsible for their decisions in life and can not blame others for what they choose to do. People tend to forget that they are choosing the path and direction of their life: no one else.
A major way in which teenagers tend to not take personal responsibility for themselves is during their high school years, where they make the most mistakes. Teenagers who make mistakes and come from a background of strict parents usually blame their parents for their mistakes by saying they did not get enough freedom. But those teenagers fail to realize that their mistakes are the exact reason they did not get freedom, they obviously would have abused it. Teens who have more lenient parents blame them for not being strict enough. For example, girls who get pregnant as teens usually blame their mom for not having her on lock down. But that is no fair to your parents because you chose to do what you did. So evidently teenagers are never satisfied with the parents they get.
When something bad happens that involves me I always know when to take personal responsibility. I know this by looking at the situation and then determining whether if any of my actions or decisions could have changed the situation. For example some people blame themselves when a family member dies of natural causes. They say things like "I could have done more" or "This is all my fault", but it is really not nothing they could have done would have saved their loved one. That is why I always know when I am responsible for my actions, because I let go the sense of over responsibility of others and there are just certain things that impact my life alone. So when I choose not to do my homework I am not going to blame my mom for not forcing me, because whether i want to go to college or not is my decision; not hers. :D