I just got back from Cancun. Supposedly the place to stay if you want to be with a lot of younger people is Grand Oasis. It's an all-inclusive and it's one of the most affordable ones which makes it good for younger people - especially for spring break and summer break.
If you want to do clubbing, I stayed at the Hyatt Regency. The hotel is very nice and it is not an all inclusive which makes it easy for you to bring someone outside of your hotel up to your hotel room. It is about 3 blocks away from the clubbing zone of Cancun, so it's also in the perfect location.