There are several guidelines - none of them are based on age.
(1) Move the child when he starts to climb out so he doesn't fall and get hurt. This assumes the child will stay in an open bed and not climb the furniture instead - or go exploring the rest of the house in the middle of the night.
(2) Find a way to prevent the climbing and keep the child in the crib until he is old enough to respect an open bed, to not climb the furniture, and to not destroy the house at 3am. While kids can get hurt falling from a crib, they can also get hurt when you are sound asleep and they are bored and exploring. Baby proofing only works if you are there to put a stop to something danagerous.
(3) Find a way to prevent the climbing and keep the child in the crib until he reaches the weight limits of the crib.
at 5:25 PM on Feb. 2, 2009