N.J.
funny, this is a very good question. My 2.5 year old still sleeps in his crib. A lady I work with still has her 3 year old sleeping in a crib (her 3rd child). We got him a nice Lightning mcqueen bed and he loves it, but you lay him down and the next minute he says 'mornin'...a few nights later he started coming into our room at 4:30 AM flicking the light on yelling 'Mornin'...although very cute, it isn't so nice to get waken up at 4:30 AM
then we moved his brother into his room, (now 10 mos. old) and our toddler would pile blanket after blanket in the baby's crib, well at this time his brother wasn't even rolling over, so we brought the crib back. I didn't want him to accidentally hurt his brother. So our plan is on his 3rd birthday we are bringing the bed back out again....and we aren't going back to the crib.
I have heard the rule of thumb is to move them to a toddler bed when they start using the crib as a jungle gym and are crawling out.