So, you think 50 something women aren't sexy? So be it. You're entitled to your opinion (and so is she).
Lots of guys think older women are sexy. I have an ex who is 57, has D cup fun bags, and she looks sexy to me. Of course, I just bought two calendars yesterday of bikini and lingerie babes, and I don't think any of them are older than 25. Beauty, and sexy, is in the eye of the beholder.
If I made comments about everyone on the planet that I had issues with, I wouldn't have time to sleep. This woman you describe is her own person, with just as many rights as you; did you ever think that maybe she thinks YOU are a complete jerk? Maybe you wear your pants too tight (or stare at her chest too often)

She might think you are some kind of perv, and that she is being too generous with you in keeping her mouth shut.
Personally, I find many types of women (of all ages) incredibly sexy. Tell her I said so.