It was fun, wasn't it? Designing your child's bedroom, first as a nursery and later as a kid's room? Well, now it houses a teenager and it should reflect that fact.
As kids grow up, their personalities emerge and they should have their own private space that reflects that personality. What better time to re-do your teen's room than the summer, when he is bored and looking for something to do? That's right, now that he's a teen, he can wield a paint brush or assemble some new furniture all by himself (or at least with minimal supervision.) Even if the room isn't ready for a major remodel, your teen can make a few changes that make his or her space seem more grown up.
Of course, it's your house, so you still have veto power. You can nix the black walls or the disturbing poster your teen picked up at the swap meet. Negotiate, compromise, and give your teen something to do this summer!