If age alone isn't a reliable factor in determining success, then why do most of the big Silicon Valley companies lean young? Shouldn't economics sort them out

If age alone isn't a reliable factor in determining success, then why do most of the big Silicon Valley companies lean young? Shouldn't economics sort them out if they really aren't getting more bang for their buck with younger employees? Yes it does - young people are easier to exploit, you can make them work long hours and pay them in stock instead of cash, and if they burn out there's another fresh crop next year. Remember that for every Google or Facebook, there are 1000 startups that went nowhere. They were full of young people too. Other countries do this too, tho' to a much greater extent. Why do you think your running shoes and electronics are so cheap...? Is that the kind of economic success we really want?