When they are big enough and aware enough to smack the crap out of you if you start to squash them maybe?
I don't know really though.
Alex sleeps with us. He snuggles up to me and uses me as dinner and a pillow rolled into one lol. Last night I got out of bed to go to the bathroom, and I came back to find him snuggled up to DH. According to DH, Alex had started feeling about for me with his arm, and when he couldn't find me, he gravitated towards DH and got comfy under his armpit lmao.
He soon started protesting though when he realised that no milk was coming from DH haha.