Somewhere around the 20 week mark, I think? We started by letting him in the bed after his early morning potty break (between 3-5 am) and when he was doing well with that (and sleeping longer through the night) he just spent the whole night there with us. Worked for us and we never had an accident in bed, but I'm also a light sleeper, so the moment he started moving around I would notice and wake up. That and I was working crazy early hours at the time, so he got an early morning potty break regardless. Now he didn't have free range of the apartment at that age. He couldn't even jump on/off the bed on his own, and we sleep with the bedroom door closed, so even now he doesn't technically have free range at night (though he'll sometimes choose to sleep on the floor when it's particularly warm, and we taught him to leave on cue when we want private time or need to change the sheets). And I still wake up if he decides he needs an early morning potty break, but that's pretty rare these days.