I was due with my second baby when my first was 19 months old. There was no way i was going to buy another crib. About 2 months before the baby was due we decorated a big boy room and got a big boy (toddler) bed and started the transition.
I didn't want my son thinking the new baby was forcing him out of his room.
It went really smooth and the new baby got the crib and the nursery.

My second son turned 2 at the end of May and he is still in the crib and is doing just fine. I NEVER use the crib as timeout or to contain him. I don't want him to associate it as a punishment. He hasn't tried to climb out of the crib yet. I don't think he is ready for a toddler bed simply because of his personality- The temptation to get up a hundred times would be too much for him

Any-who, my DS was about 17 months old when we moved him to a toddler bed but it depends on the little one.