My cousin is 18 next month and has the proper extensions that are glued in at the root - when they are taken out her hair is awful condition, they need updating constantly, and look natty very quickly, but when they are done she looks fantastic.
My sister is 17 and uses the clip in ones, which are just as good, don't ruin your hair and are easily removed at the end of the night, although can be a pain to get them in equally in the first place.
Just remember that 'permanent' extensions are costly and do need keeping up - like nail extensions, they soon look cheap if you don't take proper care of them and maintain them regularly.