The Bellevue Athletic club is nice. I am not a member but use its hotel every once in a while for basketball. It seems friendly across a lot of ages. Bellevue / kirkland / redmond is a bit friendlier to wealth in general.

For the arts / culture / charity stuff, there are a lot of fundraisers that are happy to invite the wealthy. Maybe the Seattle Foundation might be a spot to look into. I have a DAF there that I have pretty much ignored, but I think there may be social stuff like you describe.

Do you like cars? There are a bunch of surprisingly great folks in our local supercar scene, and quite a few women. It's not the stereotype you might have in mind.

The Columbia Tower Club has been actively seeking younger members. It's kind of nice to hang out there sometimes and they have some fun events. A bonus is easy downtown parking, and the club has worldwide club sharing so has benefits in other cities. It is fun for a while at least.