Mine are indoor cats as well, I take them to the vet's annually for shots and examination, then any time in between when necessary.

With an older cat I would absolutely make sure I took them at least once a year, you may find that they will want to do a more rigorous workup of an older cat, perhaps some blood tests and a few other things that are not obvious. It's always good to have base levels to refer to in case something does go wrong.