Its really up to you i put my son(turning 2 in 4days) in a bed 2 weeks ago. i felt it was time as he would shake his cot when he woke in the mornings and after naps.
so i bought a cheap bed from ikea and he loves it
Although my sister inlaw put her daughter in at 14months as she was really restless in a cot. i also know a friend from playgroup who had his daughter in a cot till she was 3.