I think that it is really child-dependent. DS is 20 months, and figured out about 3 weeks ago how to take his pants and diaper off. Everytime he does it, we sit him on the potty, but I have noticed that he is not physically mature enough to hold his pee and empty his bladder at once- he can pee 5 times in 10 minutes, all small amounts. He also does not at all get the concept of "telling" us if he has to pee or poop. But that's not to say that some kids might not be ready at this age. We're just taking it very slowly with DS and getting him used to the concept of going in the potty, but I don't expect much to happen for awhile. From what I have read, it comes more easily to kids between 2 and 3.
Re: potty training boys