D.L.
Usually they say to do it when the kids are trying to climb out of the crib because they could hurt themselves...My daughter was 2, and she never even attempted to climb out of the crib....We also had the crib that turns into a toddler bed and then a full size bed. We never used guard rails because it was so low to the ground, but my daughter never fell out. She is now three and we upgraded her to her full size bed....Just make sure to make a big deal out of it for the child when you do decide to make the transition...let them help pick out the toddler bedding etc...It will make it go a lot smoother:)