This is going to be totally based on your comfort level and your child's amount of movement when they sleep. Experts may point to an age window of 1.5–2 as a good transition point, but if your child is content in their crib, there's no need to rush them onto a bigger mattress.
Also, if your child is attempting to climb out of their crib regularly or seems uncomfortable with the small amount of sleeping space, it could be time for a move up to a kids' mattress.