T.S.
Our son's dr told me at his 18M check-up to leave him in there as long as he isn't trying to get out. At 2 1/2, he was still content in the crib, but we wanted to make the transition. He's 3 1/2 now and I swear he would still sleep in a crib if we let him.
Son #2 is 18M and climbs on EVERYTHING. I think he'll be moved way before his older brother. But, he hasn't tried yet - so he's staying in there. It's safer and less of a hassle.