Realistically, I'd say around 60. Definitely depends on physical conditioning, etc. But overall, never let your age be the deciding factor of other peoples' opinions...you know, I never let that hinder the path to my abilities.
I'm too old. 32. At least i'm retiring for a few years. Been there done that. All of it. Even with that rediculous set that deadmau5 threw down none of it seemed new to me. I'm bored with the drugs and music. After being a die hard electronic junkie for many years I think I'm done. Could have something to do with the bad taste in my mouth from this years fest, or fact that my seratonin is still not normal but I'm done.
The age when you're no longer there to enjoy the music and dance and instead start worrying about what other people are wearing and whether you are a creeper. You can be 80 for all I care just so long as you are there for the right reasons.
I remember walking out of Skylab 06' and asking two gentleman how their Skylab was - 'We came out of retirement for this one' and they were just so elated. Prolly in their late 30's and you could just feel how insanely happy they were to have made the choice to attend.