I'm roughly your age, and have been working for 10 years at the same company (in the Seattle area, not SV, but we have offices there). Unlike your position, it's never been boring - I've been able to work on both immediate-impact and far-reaching topics, and there's always more interesting things coming down the pike.

I mostly want to address the age-ism and cultural oddities issue. It definitely exists, and is worse in California than other places. However, it's a topic that can't be analyzed by averages or aggregates over a geographic area. It varies so much by company, by position/role within a company, and by individual interaction with the nearby-team cultures that you really can't decide anything based on the region. This is especially true for remote work - your cultural experience will be far different from someone living there.

So, the questions you should be asking are about the expectations for your specific interaction with the employer and coworkers, rather than about the general HR-approved culture spiel you'll get if you ask generally.