M.C.
I didn't know with mine. I was expecting a baby and 3 months or so before the baby was due I put her mattress on the floor to sleep. She was having issues staying in her room so I put her back in her crib and she climbed out. When they climb out that is a good indicator or if maybe the crib has a weight limit or something. My daughter was about 21 months when she converted out of the crib. If you need more information on what we did, how the transition went, how we finally got her in her toddler bed etc. ask me. Good luck:)