This was representative of my late-20s friends; about half of them ended up leaving the Bay Area to have kids. It's not at all representative of my late-30s pa

This was representative of my late-20s friends; about half of them ended up leaving the Bay Area to have kids. It's not at all representative of my late-30s parent friends. Almost all of us bought houses and are presumably staying here for good, barring unforeseen catastrophe or major changes in life circumstances. It's like we passed the selection filter and now we're in the 50% staying. The Bay Area attracts more aspirants than it has room for. That means that inevitably some will leave. I'd model this as tournament economics, tenure in academia, or the up-or-out culture at many big companies, though: you get lots of attrition in your first 10 years, and then after that you're in.