Being officially children (17) I obviously don't like this idea (although I won't have any stay in hotel until I reach 18). In my opinion, it would be much better solution to simply set some rules for behaviour in lounge and as long as one would comply with them, he would be allowed in lounge - after all, they all paid for it. On the other hand, those being (for example) too loud should be asked to change their behaviour or leave lounge irrespective of their age.

On the other hand, there is alcohol question which should be handled based on local customs.