That's a really interesting question; I know plenty of developers that move on to making less money. Either though Developer -> (part time) Developer, Developer -> Teacher or in one case Developer -> Raft Guide. But, I don't really know what happens as an industry.

However, I seems like a lot of officers retire early and either just retire or take a side job with higher pay and little benefits or just something to pass the time.

Honestly, I have no idea what the average is, but there are probably some interesting trends.