I don't work for a startup but just wanted to comment since I work with a lot of older developers, one who must be well into his sixties is rather interesting as, while he does have his spots where he seems a bit stuck in the past, is constantly trying to inject new technologies into the organization. We're a primarily java shop and he writes a lot of scripts in Ruby and always wants to bring more ruby into projects. I also know he has probably more of a passion for developing software (personal projects) than some of the other developers, who are younger (but still much older than me). I think its really just based on the person. If they really love building software they will keep up at any age because they see the benefit in their own projects.