Many, but certainly not all. The best developer I know started his career writing software for the original Macintosh in the 80's. He's still primarily a developer.
Here's the important question: how many twenty- or thirty-something startup founders hear that and think "dinosaur who writes GOTO statements" (false), versus "awesome hacker who has been constantly seeking out the newest technologies for 30+ years" (true)?
Once again I say in a start up I want experience. I want developers that can deliver with minimal management and who GTD rather than work twice as long to deliver tightly coupled, dependency riddled, fashionable code.
Also another point to your comment. If the founders are 20 or 30 something, is that why they are only hiring 20 or 30 something developers. It's their contacts in their network, previous co-workers and friends. Perhaps the issue with start ups in that not that higher percentage are started by 45-50 somethings. Hence it would be interesting to see the split of developers ages compared to the co-founders age.
Here's the important question: how many twenty- or thirty-something startup founders hear that and think "dinosaur who writes GOTO statements" (false), versus "awesome hacker who has been constantly seeking out the newest technologies for 30+ years" (true)?