My personal policy is no enhancements on under-16s. Like Vannessa said, at the age of 16 and under you are doing I think 2 hours of compulsory PE at school per week - hockey, netball.... which I know from personal experience (temporarily lost a thumb nail and have two permanently mis-shapen nails) wreak havoc on young hands. I personally never wore my nails much longer than 1 or 2mm over the end of my fingers for that very reason, and all the girls that did were forever breaking them.
For a 15-year old in their last year at school (so they will be turning 16 but not quite yet) I would apply enhancements once the school year is over, but not before.
I will do weekender nails for special occassions but the price for those includes removal. For this reason I've produced a 'teen menu' which then only includes the treatments I am prepared to give young girls. I've tried to tailor them more to their market so they feel like they are getting what is best for them for their money rather than paying for, say, anti-ageing treatments or such like in their manicure
I would much rather give a girl a series of my 'nail biter no more' manicures and get her own nails looking lovely than perform what I see as an adult treatment on a young girl.
It makes me sad to see such young girls so preoccupied with their appearances.. clothes, nails, hair, before they even become teenagers. Every client will be different and I hope I haven't offended anyone here, but my personal feeling is a blanket 'no'.. at least then I can occassionally view a case independently and make any 'yes' decisions the exception, rather than the rule. At the end of the day I know that none of my local salons (thank god) are NNS, so if they do go to them and get a yes, at least they should take care of their nails even if I don't agree with the decision.