If you are willing to share, I'd be curious to hear more about your situation. I've seen salary plateau at certain career levels and ages (40s usually), but saying salary goes down at all (let alone sharply) is something that I expect is not a general truth about the market at large. My anecdotal data comes from ~20 years in recruiting engineers.

There are several reasons you could see a salary drop (stagnating skill set no longer in demand, move to company that pays less, move to industry that values programmers less, etc.), but age alone isn't likely to be one of them.