There is no catch all age/length for what size prey you should feed. Its all about the girth of the animal and if he/she is able to handle the prey or not, in my opinion. If your bp is able to take rats at 20 inches, go for it. If it takes your bp a bit longer on mice to get big enough to make the transition, its ok too. You've got to remember, they are all different and grow at different rates.
yeah what he said.

roxy is pretty girthy for a 20 incher so rats are perfect for her but for somebody elses snake with the same length but not as much girth it wont work and you need to feed something smaller (mice probobly)