You can check with your vet if you’re unsure. Most kittens should remain on kitten food up to 12 months. Larger cats, like Maine Coon’s for example, reach maturity later so shouldn’t move to adult food until 18-24 months.
When transitioning to adult cat food, do so gradually over 7-10 days to avoid any tummy issues. It’s normal to see some stool variation during this time.
Every cat is different so take things at your cats pace.