Are we talking about your 3 or 7 year old? My 4 yr old started washing herself(at least she thought she was) at 3years old. I would assist in the lathering on a sponge, and remind her where to wash. Ever since I can remember, my daughter has been taught how to wash herself. Even as far as me saying ....wash those legs, as I would be washing them...she would follow with, "washa ams mamma". Now at 4, she is taking showers, and washes very well. There are times when I still have to remind her, or if unsure, I have her re wash herself, and I will assist. Every child is different. I love Denise P response about her son will allow her till prom. :)
Even though my daughter is washing herself, I never leave her alone in the shower....I leave the curtain 1/4 of the way open. Sometimes I even shower with her, so I can visually see how she is progressing in washing herself. Thankfully, my daughter has always been the, "do it herself girl". It is hard to for me let her do it sometimes, but good that I am trusting her to do it. Although my friend has a 6 yr old, and he is just starting to wash himself throughly. It all depends on the child, and how well we trust that they are doing a great job. These times are proper important in teaching them hygenine.