Age 6 for us. When I suggested that he try at age 5, he freaked out and I backed off. At age 6 he asked "what if I get some on my hands?" and I explained that's why we wash them!
Since he wasn't internally motivated, I did a sticker chart.
Week 1 = we wiped and he wiped after us, just to practice the motions without having to worry about getting the poop off.
Week 2 = we alternated wiping - he started, we wiped, he wiped, we wiped
Week 3 = he wiped himself and we 'checked' to make sure he got it all.
At the end of each week, if he got 6 stickers, he earned a bus ride (he's majorly into buses). At the end of the 3rd week, he got an extra long bus ride that he'd been wanting to do for a long time.
It's been about 6 weeks since we started the new regime and he's fully wiping himself now. He still puts a sticker up on the chart, even though he's no longer earning anything.

It must mean something to him.