Can't really base prey sizes off age. Snakes are just like people and grow at different rates. One snake at 10 months might need this prey size, another might need that prey size.

The easiest (and I think the best) is to feed it something that is 1.5x's the widest part of its body. So if it's the width of a pinky finger, then feed it something thats 1.5x's as wide. Probably a large fuzzy or a small hopper. I bump up to the next prey size when the lump is no longer visible. And every 7 days is fine for a schedule.