1 - Writing code is a young man's game. Older devs get one too many platform/language iterations behind the curve so can't apply for as many jobs - obviously not true for everyone, but could account for something
2 - Older devs are tempted into management, and so write emails for a living instead of code
3 - Older devs may have more commitments outside of work, so can't take on jobs that demand long hours in the office and long hours at home keeping up with tools and trends
4 - Huge numbers of people are starting to realise you can make good money sitting on your arse all day and writing code, so there are lots of them applying for jobs with low experience - that's not to say many of them will still be trying a few years from now
5 - All the best experienced older devs don't apply to job ads, they just move easily between contracts using their network