Personally, I only advocate for vaccinating kittens as soon as possible (which is about three weeks of age). Especially to avoid the dread that is panleukopenia, which can decimate an entire litter in a matter of hours. Unless your adult cat is living alone (as in without other cats or animals) in a very sterile, clean, human only environment, and/or is in direct risk of some specific diseases (an outdoor cat in a rural area can very easily catch rabies for example) I generally advice against yearly boosters because of the risk of cancer. Some vets are really biased when it comes to vaccines because is a good source of income for them, so I personally do not trust their input on vaccinations. Though to be honest, I don't trust the local vets in general. Anecdotally speaking, being a cat owner for over 20 years I never had an adult cat come up with an issue that could be countered with a vaccine, but my cats spend at least some part of their lives, living in a not very hygienic environment, either because there was always more than one of them (you can keep a place only so clean with five+ cats roaming about) or were street rescue dumpster divers. I have a 12 year old cat that has never seen a vet in her life so far (I just knocked every wood surface in the house) and is obviously not vaccinated, she spend the first 6 months of her life in the streets which probably did wonders for her immune system. But again, anecdotal, at the end of the day is up to your discretion, but keep in mind the environment the cat is in, and remember just because a place looks clean and smells clean, it doesn't mean is 'clean' bacteria wise.