ROK = year (as a singular noun)

You use rok when talking about a specific year or one year of time.

Examples:

1. Jaki jest rok? — What year is it?
2. Rok 2025. — The year 2025.
3. Mieszkam tu jeden rok. — I have lived here for one year.
4. To był dobry rok. — It was a good year.

LAT = years (when talking about age)

You use lat ONLY in the phrase how old / age.

Examples:

1. Mam 5 lat. — I am 5 years old.
2. Ile masz lat? — How old are you?
3. Mój syn ma 4 lata. — My son is 4.

This is a special grammatical rule just for age.