It's not a question of age really as all snakes grow at different rates.. more of size.
When they can take a substantial sized mouse. I feed my two a large mouse each fortnight, but I started feeding my Blaze, a small mouse, every 10 ten days.
After they can comfortably take 2 fluffs for a while try a small mouse. If he takes a small mouse well without strethcing too much (seeing too much skin between his scales) and leaves a noticable "Lump" not a "bulge" then feed him a small mouse every ten days.
this way he avoids getting obese as the meal size is larger than his fluffs, but the time is more spaced between meals.
it all depends what size food you are giving them too, eg. I feed my adults one medium every week but if I was to feed larger it would be longer between meals