other HN threads have debated heavily about why the startup industry is centered in SV, given its ultra high cost of living and labor. and yet, despite everything, SV remains at the center.
and that's why i think your explanation is closer to the truth. higher salaries for older workers wouldn't matter. what does matter is that management thinks older workers will bring the wrong sauce to the product, exhibit reasonable skepticism of the (probably unsuccessful) product concept, generally fail to buy in to the vision and sass their 24 year old masters.