I have known Europeans that were done with this by 6 months. It was all about scheduling - having & maintaining. So age is not really an issue. IMHO
With that said having been a daycare provider, we charged extra for non-toilet trained kids - diapers = extra work - no brainer if you ask me, so I never cared one way or another.
But I will tell you routine, status & treats can not be underestimated. When I did daycare my mother lived next door & was doing the same. During the summer months every afternoon M-F I would take all the potty trained kids to the local pool & all the "diaper babies" stayed home for naps. My parents were always shocked how quickly they learned, I had one that decided on his 2nd birthday that he was done with diapers ha came back after Xmas holidays & told me oh so proud that he was now a BIG kid, I let him skip naps that day & he fell asleep on my brand new leather sofa & had his one & I believe only accident
. NO I did not get upset with him, it was MY fault not his, what are you gonna do?
Incentives, routine, routine, routine. The routine itself doesn't matter so much just that the same things happened @ the same time day after day. The only part of the routine that I will get into was this:
before every meal they had to wash their hands, after every meal they had to go potty, wash their hands & brush their teeth, if they were a "diaper baby" they were allowed to start doing 1 thing after another themselfs for instance at first I would to it all, then as they progressed they were allowed to wash their own hands,brush own teeth, etc I would only praise BUT I had the older kids to "inspections" - they had to meet the standards of their peers....I am sorry but standards are just that.