My BPs are on rats that moment I get em. Honestly, I try not to go with age as a measure of what they should be eating as the BPs vary so much in size as well as their affinity to eat different size prey. I typically stick to the feeding of prey that is 15-20% the size of the snake, but even that varies. For example, I fed 2 of my BPs Sunday - one is a 300g male the other a 750g female, they're almost the same age (female is a couple months older than the male). That said, they eat the same size prey (rats in the 70-80 gram range). The bigger female won't even look at prey that is 20% her size, the male devours smaller prey. So, I guess to sum it up, my advice is typically to use weight as your benchmark and adjust to your snake's preference.