We told our son at age 13, and really only did it because he would HAVE to be part of the IEP process at the end of the year. We hesitated to tell him because he hadn't asked questions, and we didn't want him to feel different from other people, AND we didn't want his diagnosis to be used by him as a reason to not accomplish whatever he could accomplish.
When we told him, it was NO BIG DEAL. We were amazed at how low-key it was. He asked a few questions, and that was it. I think it had little impact on him. A couple of times at school, he has talked about his diagnosis in class, somewhat inappropriately, and that was what we were concerned about -- kind of using it as an excuse when he didn't feel like doing something. But he doesn't do this often. He just doesn't currently have the skills to realize when he should, or should not, talk about his diagnosis. Talking about it is not a problem unless the group is not a receptive or understanding group, and right now he can't distinguish between the RIGHT situation to discuss aspergers, or an unreceptive or socially uncool situation to discuss aspergers.
Every once in a while, he sits down with us at night and starts a discussion about aspergers. It's always very fun to talk about it, and I feel like all of us come away from the discussion feeling better about our son's future.