My wife is ten years my senior so I'm not exactly unbiased when it comes to older women
. But with that being said, I do think that not only has she gotten sexier over the years, but when she was in her forties she was likely at her peak.
Not coincidentally that decade was when we first got into swinging and when I think she really had her sexual awakening. Whether being in her forties made her more open to the Lifestyle or if the Lifestyle caused her to be more open is perhaps debatable, but just the fact she went from the sexually shy woman she was in her 30's to someone who suddenly took like a duck to water when it came to having sex openly with other men and women was like night and day from what I had come to know of her.
Most of the other guys were closer to my age and thus being able to have sex with women in her forties seemed like something they very much looked forward to as well.
As she has only maintained her high libido well into her 50's and now 60's; as long as the physical ability and need is there, I don't see any age being the limit for sex.