Developmentally, children usually are making the connection from brain to bladder by age 2 3/4 - 3 years of age.
It's hard to potty train sometimes. I've seen (from a childcare provider's point of view of 28 years) parents say their child was potty trained but when you ask if they are getting them to the potty on time they say yes. That tells me it's the parent who is trained.
There is some great info on this on this site...http://www.zerotothree.org/baby-brain-map.html
Also this site..http://www.ask-nanny.com/child-development.html but most of all with potty training...http://www.aap.org/healthtopics/toilettraining.cfm
I'm trying to remember the site... American academy of Pediatrics there is a site I bookmarked that actually says what a child normally can do by age 2 1/2 on out. By the time they are 3 y/o they can plot a course and plan how to put a toy down, go potty, dress, wash hands and get back to toy. Interesting.
at 8:05 AM on Jan. 26, 2011