I am often asked, "when should I take my child out of their crib and put them in a bed?" I really think that there are several reasons to transition a child from their crib to a "big boy or girl bed".
The first and foremost reason seems to be when your child is climbing out of their crib. I have had many a patient (and a few of my own children) who will attempt to climb out their cribs one time and then if seems as if they were frightened by the experience and that was the end of it.
On the other hand there are some children who are adept climbers and they will routinely climb out of their cribs. In this case, due to safety concerns, I think it is necessary to put the child (typically a toddler 12 - 18 months), into their own beds. Now some of my patient's parents have purchased a crib tent rather than even think about going to the "big bed"! They don't even want to discuss beds!
I think another compelling reason to transition a child to a "big bed" is when you need the crib for the next baby. Call me frugal, but it seems crazy to purchase another crib when you really won't need it for long. It is possible to put a 15 -18 month old to bed in their "big boy" bed and they will stay put! With that being said it is also possible that the child will decide they just want to get out of bed on their own, but this problem can occur at any age, and requires behavior modification.
I remember putting our first born son in a twin sized bed that he helped "pick out" when he was not yet 2 as I was expecting another baby. We also let him pick out his sheets for the new bed (I think they had Big Bird on them) and he easily transitioned to the bed. But, when he turned 3 years old he decided he wanted to get out of bed and "rejoin" the parents and he was difficult to break of this habit!
The last reason to transition to the "big bed" is when the child is either just too big for the crib (or breaks your back trying to get them in there), or when you as a parent are ready for the next step. For most of my patients this occurs when their child is around 3 years old, and the crib goes away and the big boy/girl room starts.
I do have patients that are over 4 years old and still in a crib, but that is the usually the exception. One cute mother comes to mind who has twin girls (her youngest). When I asked why they were still in their cribs she replied, "I just can't find the right bedroom furniture" which made me laugh. We all have our reasons! Those cute girls are now 6 and she finally found the "perfect" beds and they are happily tucked in each night.