I suspect the main reason MS or Google or anyone else prefers young programmers is that they tend to be more committed and 'fresh', in the sense that they are n

I suspect the main reason MS or Google or anyone else prefers young programmers is that they tend to be more committed and 'fresh', in the sense that they are not yet bitter, cynical, and hopelessly opinionated. They also tend not to have as heavy a set of personal/family obligations, so can work cheaper, longer hours etc. And of course on average, if you're too lazy to actually find out from the interviewee, they probably tend to have more up-to-date skills. OTOH, a seasoned veteran often has a knowledgebase in a specific problem domain that can be immensely valuable. However, it takes a lot of work to find the right person and ensure that his KB is the real thing, is up to date, and that the personality attached to the KB is going to mesh with the culture of the rest of the team. When things are going gangbusters, it's easier to just ignore these subtleties and hire the lusty, naive young waifs who will work their skinny little asses to the bone for da man before they realize their souls are being sucked dry by evil sorcerers.