M.M.
We converted when DS starting climbing out on his own.
We did this at 23mos, and it was tough. We lock him in his room to this day. He just won't stay in his bed.
I personally want to do this earlier with my daughter. I think if she's young enough that we can "teach" her to stay in her bed at bedtime - rather than waiting until escape is attempted - it'll be easier. That's just a guess. DD is 15 mos right now, I'm going to try this around 18mos.