I think you are missing a small detail. Older people tend to be more risk averse and more likely to stay in their current position.
More than likely, the answer to the question, "where are the developers with 20 years experience?" The answer is, "the same place they were 10 years ago."
More than likely, the answer to the question, "where are the developers with 20 years experience?" The answer is, "the same place they were 10 years ago."