For one of my little ones she was almost 1.5 years old. I couldn't bend over to put her into the playpen because I threw my back out. The parents then wondered why she wouldn't sleep in her crib at home! It was because she was sleeping in the middle of a double bed here! lol oops!

I just put a now 2 year old on a cot, as he has been sleeping in a big boy bed since December. It was just a matter of me getting another cot.

For me, it's just a matter of when they're ready, how big they are (when I can no longer lift them out), and what's going on at home!