Berlin Tour Guide who is good with teenagers?

I would check out Jeremy, the Berlin Expert. He is very personable as well as knowledgeable and he usually brings his cute little dog along if his guests are ok with it.
http://www.theberlinexpert.com/

If that doesn't work out, you might contact [email protected]. She's a young British expat who also leads tours for Rick Steves.

How about a guide who was the 15 yo son of an East German bureaucrat when the Wall fell? Robert Summer. He has some nice stories about his experience of the Wall falling, his first trip into West Berlin, and the capitalist product he held out on the longest before trying.

You could also check with Lee Evans, who helped Rick write the Germany guidebook.
http://www.berlinandbeyond.de/