In the past I had a pair of littermates who I'd adopted at the age of 10 weeks. When they were around 12 years old I asked my vet how old would be "too old" to adopt an additional cat -- that is, at what age would they not be able to tolerate an addition -- and she said "pretty soon". I had the impression that she was making this judgement based on their age and not an evaluation of their individual personalities.
My current cats are middle-aged (about 6 and 8), and while my spouse has informed me that we have no plans to adopt more cats, it got me wondering about my vet's past advice. Is there such a thing as "too old for new additions"? If so, when is it in general and how can I tell if my cats have reached it? How relevant is the age of the newcomer? (I can imagine arguments for getting another older cat and arguments for getting a non-threatening kitten.)
For purposes of this question, assume that all cats involved are basically healthy and well-adjusted indoor-only neutered cats.