I started my son when he was 7, supervised. His aim in the porcelain throne was so awful. I wanted him to know what it felt like to clean that stuff.
He could manage the sink and the bathtub - no problem. I think the commode just grossed him out, which was great. As soon as the job became his, his aim got a lot more precise. If he missed, he got out the cleaner right away and cleaned. It was his bathroom, all stains belonged solely to him.
Unfortunately, my cleaning habits didn't take because I went to his apartment and went to the bathroom just to check it out. I swear I thought I was walking into a Texaco station. It was a scary movie, to say the least. I made him clean it so I could use it. Naturally, the roommates are the pigs, not him.
For Pete's sake....start him young...when he's old enough to squeeze a sponge!