What I have been told is that as soon as the child know how to climb out of the crib it is time to change to a bed. As the other mom said it only takes once to get really hurt (even if you think your child only does it when you are in the room). I found out the hard way that my daughter could climb out of the bed and she ended up with a huge bump and scrap on the top of her forehead, over the nose down to her chin. Thankfully no stiches were needed but scared the carp out of me.
I think we changed her to a bed shortly after her 2nd birthday. If your child was not climbing out I would say it is fine to wait no rush if everyone is happy but with crawling out I would lean towards switching to a toddler/big kid bed very soon.
My daughter moves all over the bed so we have it against the wall and a bed rail which seems to help keep her from falling out. So if that is a worry you can always make it work to help your child feel secure and safe.