What is tenure/turnover like in the big SV companies? I see salaries rise pretty quickly and the ceiling is fairly high, but does this simply mean you're like

What is tenure/turnover like in the big SV companies? I see salaries rise pretty quickly and the ceiling is fairly high, but does this simply mean you're likely to rise to a certain level and then get replaced? For 40-year-old techies looking for a company to round out their career, are the SV biggies the wrong place to look?