Like with any animal, this can depend on the particular bird. That said, roosters are sort of like human males: they will probably be fertile most of their lives! And if you have a great rooster, it makes sense to allow him to fertilize eggs for as long as possible to pass on those good genes.
Black Frizzle Bantam Rooster
If you're worried about whether your rooster is fertilizing eggs, just check your eggs to see if they're fertile. If they are mostly fertile, your rooster is doing his job!