I've been working at a bunch of companies in my career and I'm well over 50 and now have decided to work only for startups until it's quitting time in a few years. It's like riding a roller coaster - after one exits, I just want to get back on the next one. One before this, Flip Video, was great, but we all ended getting laid off. In my subsequent job search I encountered a fair amount of age discrimination in hiring by companies large and small. However, at the startup that I currently work for I didn't get that at all. The interview was as tough as any other, but I didn't get the some of the negative vibes I did at other interviews. When I joined, I noticed that there were a fair number of crusty old guys like me, right along side of the kids right out of college. The common thing about us is that we are all good at our jobs. The company has no problems with terminating people who can't deliver.
Discrimination exists, but not everywhere. The smart companies realize that discrimination in any form cuts down your pool of talented people and is therefore counterproductive.