And the salary offered begins to go down at 40 too. Sharply after 44 or so.
Where I am now, at 51, I have had to learn to get by on what I was making as a SysAdmin 20 mumble years ago. But at least I have a job. Before getting my current job I was having to learn to get by without a place to live too.
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.
I was severely burned in an automobile accident (60%, 3rd degree) on June 6, 2006 -- and was completely unable to work for a couple years. It took a little over 4 years total from the date of the accident till I got another job.
Every job since then has been with a startup.
So at this point, while a 4-year gap will still be a downward multiplier to some, the effect will be relatively mild.
Better to focus on the the sheer inner strength it must take (which I, and almost all these people summarily rejecting you, can only begin to imagine) to pull off a recovery like that, and how that makes a much more solid person overall -- and hence, a way better hire.
Note that it's all based on perceived value, which is only partly based on actual value, which is only partly based on skill. I'm not trying to brag or put anyone down here. A lot of it has to do with what I said in another comment about specialization. Nobody wants to pay a thirty-year veteran's salary for a job where they don't already have deep expertise. People who can find jobs closely aligned with their skills and specialties often continue to do OK. People who can't - and it's usually more to do with shifts in the market than in them - get left out.
The sad fact is that many older programmers' skills and instincts often do apply outside their area of obvious expertise. Recognizing code smells or knowing how to test code properly are valuable skills in any specialty. So is communication, or scheduling, or being able to recognize and mentor up-and-coming young developers. Most older programmers - not just the few lucky ones - are well able to justify their salaries but never get the chance. That's a "soft" kind of ageism, but it's ageism nonetheless.
But again -- only to a certain degree. And more so: there a different ways of looking at it, and more importantly, certain things you can do about it -- such that if you groom your skills carefully, and pick the roles you apply for carefully, also -- you can turn your age into an upward multiplier, rather than a downward one.
That's my quick take. The main thing is, please don't give up hope -- there's still a lot one can get out of the very substantial skill investments one has made by that age, provided we don't fall into certain traps (perhaps the most insidious being: believing all the crap that SV and a good chunk of the industry believes about the magical advantages that very young hires have over older ones).
Am swamped for time at the moment, but perchance, may get back to this topic in a couple of hours.