Have you read The Lovely Bones? The MC was on the low end of this age range. The third act of the play Our Town also has an interesting perspective on the attitudes of the dead. I'm afraid we can't tell you WHAT to write, so this needs to be fairly generic.
Your dead don't need to have identical motives as the living - they can be different depending on what you aim to achieve in your story. And of course, human motives are diverse. Suicides would be an obvious start. Obviously much youth death is sudden/violent. Teens have the same motives as the rest of us, but the dead can't achieve, only regret or atone.
For some teens, life is a pleasant haze of drifting through life until confronted with the challenges of maturity. That doesn't make for much regret. Troubled teens suffer terrible angst, and struggle with being brave enough to do what they truly want or be the person they really are inside. They often hurt their loved ones in the process of trying to become adults and differentiate themselves from their parents/role models. They may struggle with sexuality at this age. They may be struggling to even FORM an identity, and are influenced strongly by peers to define themselves in ways they have little control over. Intense pressure to fit in leaves them angry, frustrated, surly, and confused.
So basically, you need to get inside the head of a teen and try to think how they would feel. Imagine if each of the characters in The Breakfast Club died on the way to the school that day - what issues where they dealing with, how did they respond to those challenges, and what would they have regretted?
As to HOW these folks can guide teens through their regrets, you probably need to get a guide to adolescent counselling. The same skills and techniques a therapist would use to guide the living through their feelings are the ones you would need to fix the dead - unless the rules are radically different, in which case only you can know what they are.
If your folks can influence or communicate with the living world, the possibilities expand to actually fixing some problems. So you can imagine a thief apologizing to a victim, a big sister protecting a younger one from abuse, or (more darkly) an angry teen causing the death of their killer.
How do you want it to play out? The regrets of your dead teens will have a lot more to do with the spiritual, theological, and emotional issues you want to explore in your story. Do the counsellors make mistakes and have their own regrets? Do they guide the lost souls to moral perspectives, or do the dead have different motives? Is your afterlife a kind of second chance to either turn dark or atone for sins?
The choice is up to you.
