When he is big enough for them, go for an adult mouse. Are you picking out your own over at the store, or do you get f/t? If you are picking out your own, i would get a quick eyeball of how big Link is before you go to the store so you can get the correct sized rodent. There is no catch all age/length to start feeding a certain size prey item ... its all about the girth of the animal in my opinion.