I agree with all the advise given above!!. As for which discipline i.e powered or gliding I don't think an eighteen year old would mind either. I do both and find them both as equally rewarding and different. Gliding might be the cheaper opetion and if you go down this route try to arrange it for a sunny THERMIC day to get the most out of it. If you go powered he will not doubt love this but be prepared for him begging to use your wallet to gain a PPL. Happy looking