I tried with DS around 18-20 months, before DD came. It was horrible so we switched back. I ended up trying again around 2 1/2 and went so smoothly. He just wasn't ready that early, but I know some kids are.
We switched my older DD at 18 months and it went great. She had been SSTN since 2 months. She had been napping on a cot at daycare since a year. My younger DDD (14 months) is already ready for a big girl bed (crawls onto her sisters all the time and tucks herself in) but we are waiting until the fall to move her as I just think its too soon to do it now.
Re: switched to big bed early?