I think with any growing kids as long as you are talking healthy foods like fruits and vegetables let them eat as much as they want. Just encourage them to be active too usually not a problem with kids and teens!

Make sure there are plenty of healthy snack choices in your house, not just junk food. If they are eating a well balanced diet and getting plenty of exercise then I wouldn't worry too much.

How much food is enough?  Teenagers can eat lots of food.  Sometimes I worry that my child might be eating too much.  However, they are not really heavy.  Just not really skinny – if that makes sense.  How do you monitor a teenager?  They have such voracious appetites!