How do i get my cat 3yrs old to stop eatting my younger cats 9months kitten food?

Ive had my 9month old cat for a month now and my 3 year old cat is starting to eat the kitten food instead of his own adult cat food. I leave food for them in their bowls all day because they eat at different times of the day. I cant stop the older cat when Im gone at work all day, only when I catch him.