Tooth enamel is the hardest surface in your body... yes, even harder than bone. The only way to damage it is to actually break it or chip it. With that being said... whitening your teeth does not damage your enamel. People frequently report sensitivity with whitening procedures, though, especialy if you get the product on your gums. You can avoid -or at least reduce this- by wiping any excess product off of your gums and by using an over the counter fluoride rinse for 2 weeks before you do the whitening. The fluoride will help reduce tooth sensitivity, but not gum sensitivity. Lastly, it is fine for a 14 year old to whiten their teeth (with their parent's permission) as long as they don't have any primary teeth left. I hope this helped you... good luck!