Why not? If you don't need the crib for another child, and you're not worried about him hurting himself, or breaking the crib, why not leave him in there if he's happy? If you're worried about something, try moving him into a toddler bed. It's the same size mattress, so if he likes the cozy enclosed feeling, maybe that will be good for him. But if you don't have to move him, and he's happy where he is, then keep him there. My kids were all out of cribs by 18 months. But that's because I needed it for my second (second and third, actually - twins), so my daughter was out at 18 months. My twins were out at 15 months, but that's because we were moving and needed to get rid of one crib. My youngest was out at 12 months, but that was because she kept getting arms and legs stuck in the slats, and she was not happy in a playpen because she wanted to be big like her older brothers and sister. (seriously... she followed them around everywhere trying to do exactly what they would do). But she was happy that way. She's on a twin mattress on the floor (no box springs) and does really really well. If I had my choice with all of them, I would have left them all in cribs until I couldn't. I liked knowing they were safe and confined at bedtimes. They couldn't get around the room or out of the room. Anyway... long story short: Why move him if you don't have to?