I'm not sure for others. We took off the front rail and converted it to a toddler day bed right around DD's 1st birthday. She was just consistently climbing down off of couches and beds the correct way supervised and it's just about a foot and a half off the floor. With a safety rail in place, we thought she was ready. I think it's more about your comfort level.
We moved DD to a twin bed at about 21-months simply because we need the crib for DD #2 who is coming soon! We would have kept her there longer if we could because she hadn't started trying to climb out or anything.
I am highly considering doing this soon. DD is a monkey and climbs on everything. She is already trying to put her leg over the rail in her crib and gets very very close. I am scared to death she will fall. I am dreading the transition, but would rather be safe then sorry.
Re: Toddler bed question