Mostly it has to do with the parents. If the parents don't treat them like a "big boy" you will have more trouble getting him to do it at your house - not impossible but definitely harder as in he will hold out as long as possible. WIthout the practice at home it will be harder too.

Normally I wait until they have turned 24 months before I worry too much about what they can and can't do as all develop at different rates too. Between 24-27 months I assess independence skills because I make it clear to parents that I will not begin potty training until some of these things like pulling pants down is learned which spurs them to realize the child should be doing these things for themselves.