N.B.
This is going to sound weird, but my son told me! I don't remember exactly how old he was maybe almost 3 and one day he said "mommy I'm too big for this bed". My husband and I jumped all over it, we found a toddler bed for $25.00 and put him in it that night. The transition was smooth, we took down the crib and put it in the closet. I was going to leave it up but thought that would cause some anxiety so we took it down immediately. He took right to it!
Hope this helps!