Eldest i have put a cat through a dental is 13, but there are cats on here that have been older when they have had an op. I think as long as they have pre-anaesthetic bloods, and a drip during, it should be OK, and I personally would go down that route rather than antibiotics, as bad teeth can affect some of their organs. It isn't true that soft food is better though, most of mine have preferred dry with bad teeth.