DS was 13 months when he got his big bed, tho he still spent a lot of time in our bed (gave up the cot at about 8 months lol).

We decided he was 'ready' as soon as he knew how to climb off it safely. So he got his big bed and a bed rail. We still have a bed rail, altho I'm not sure whether we still need it. He throws himself into the wall at night (can hear him banging around) so I guess we'll just leave the rail on the other side for a while yet.

Just be prepared for them to get themselves up when they're in a big bed. They're not confined to it like in a cot, so it could change how you put them to bed etc. We didn't have any problems, but I can see potential problems for self-settlers. hehe.