I think this is like asking at what age should a child sleep though the night, be out of diapers, stop nursing or using a bottle, or the other host of "milestones" that kids are supposed to hit. I don't think there's any universal age because there are so many variables involved. Just like everything else, I don't think anyone is going to go off to college without knowing how to wipe their butt.
My daughter is going to be five this fall, and her dad and I still wipe her because she doesn't want to do it herself. Also, she has extremely sensitive skin, so it's important she be wiped really well. None of this has been a problem because we don't mind, and she hasn't been in school or daycare, so no worries there. However, she'll be starting preschool next month and even though they're willing to help somewhat, I think she should be able to handle things in the bathroom by herself while she's there. So we'll be spending the rest of the summer on butt wiping lessons.
But if she still wants me to do it at home, no problem.