My oldest just turned 6, and he takes care of it himself 9 out of 10 times, but occasionally needs help and asks for it, when things are a bit looser and he, uh, slips... (sorry!). I rarely find any stains on his undies. My almost 4yo has started to try himself but it's more for independence - nothing actually gets wiped away, so we have a ways to go - which doesn't surprise or concern me.
My oldest saw an OT for awhile (not for ass-wiping

and I had just asked her incidentally and she said 5 / Kindergarten was the general timeframe when you would want to help them get some basic proficiency with it.