I don't think there is a 'usual' age and I think it's dangerous to look for one

My DD was 24months when she TT completely and not a wet bed ever. DS1 however is 5 and although he's not in nappies anymore (only in the last 6 months he stopped) he has a wet bed 4 nights out of the week. It surprised me as DS1 is a very intelligent kid and he's always not been a problem to learn things but bladder control is NOT his strong point