Sleep sacks to blankets

Just wondering when people went from putting their baby in a sleeping sack to giving them a regular blanket?

My son is 20m and is still in his sleep sack, but he’s recently taken to sleeping on his front with his knees bent up- he will fall asleep like this quickly when he’s not in the sleep sack and stay like it for ages, but when he is in the sleep sack I’m not sure there’s really enough room for him to do this as he wriggles around in bed a fair bit and always ends up on his back.

Is it time to start using blankets? I’m not sure if he wriggles because he’s not comfy in the sleeping bag or if he’s just got ants in his pants so I’m dreading the day we move to blankets in case he wriggles his head underneath 😕