I've had sleep study twice, mild sleep apnea. am 18 years old, what underlying problem could i have? does it surprise a teens on cpap? not overweight!

Upper airway struct.: Your upper airway is the most likely cause. Large tongue, asian descent, shape of palate, shape of mandible, uvular size, and many other facial characteristics can all contribute to sleep apnea. Your sleep doc should have evaluated you for these traits. Ask your doc what he or she thinks may be the problem. Even skinny little kids can have apnea! best wishes.

Depends...: If you're 18 and not overweight, obstructive sleep apnea in your case may have numerous possible causes. You should see an ENT doctor for a thorough evaluation of your upper airway anatomy to identify your possible areas of obstruction (e.g. Base of tongue, tonsils, adenoids, deviated septum, dentition, jaw position). Everyone is different so you should get checked out.