Well, I think about 2 years old would be old enough to stop. I don't let J (age 37 months) take a bath by himself but I start the water, add bubble bath, and let him climb in. He plays while I sit in the bathroom on the toilet lid or a small chair. Sometimes we visit or play but usually I work in my puzzle book or read, something to make it less boring for me. When I am ready for him to get out (usually about 15-20 minutes) I have him pull the plug and stand up. I keep the kids shampoo out of reach so he won't finger paint in it on the walls or something messy so I get shampoo in my hands and wash his hair. I help him scrub any parts not clean enough and then we use the hand held shower head and to rinse him off and get the suds out of the tub at the same time. He climbs out onto a towel or mat and gets dried off. He can stand on the toilet seat lid for you if you can't bend as much right now.
K is 6 and I let her have time in the bathroom alone during bath time. I leave the door open and never go out of hearing. I usually find a task in the hallway or sorting laundry in the next room. Nearly all of her friends take their baths and showers by themselves at this age. They have been for years and think it's odd that she doesn't. I had an extremly hard time giving her this space. I had a friend die in the shower while in high school by falling and hitting his head so it was very hard for me to let her have this space.
I have to say I think it's past time and getting to have more independance is a growing experience for him and you. As long as you keep it safe for him then start letting him have his own (supervised)bath/shower time.
Children learn while they play and water play is very important. They learn the feel of water, how it slips through their fingers, how it feels on their naked skin, that they can make bubbles by blowing on it, they can pour it into cups or through water toys, some things float, some sink, and lots of hand eye coordination. It's my favorite time of day because I have to wind down and spend time just doing nothing to the house, or planning anything, etc...it's my special time, alone time, with each child.