Will I stop being employable?As a conventional, salaried programming employee? Yes. (In what other field does 5 of so years of experience allow you a title with "Senior" in it?)
My experience is illuminating in that I'm blessed with genes that allow me to look much younger than I am. I'm now 50 and in casual encounters (e.g. in stores or restaurants) I'm still routinely mistaken for an 20s something possible student (of course I dress the part).
In the '90s, as I made the journey from 30 to 40 I noticed it became harder and harder to get results from sending out resumes ... until I got a clue and scrubbed my resume of the hard evidence of how old I was (education dates; by the time this happened I'd already scrubbed it of jobs prior to 1990).
The difference was like night and day, and if I watched my words I was able to keep up the subterfuge until I was hired and had to prove to HR I could be legally employed, something that didn't necessarily make its way to my managers. There was one interview a bit less than a decade ago where I slipped and mentioned working on PDP-11s ... the response was "Just how old are you?!??!!" (yes, that's illegal, but it was an instant honest response). They did hire me but it didn't work out; they were desperate but it's just as likely it was due to total cluelessness of the real customer, a state agency.
So my advice is one or more of the following:
Hide your age if you can and for however long you can. They can't legally ask.
Transition as quick as you can to a field that doesn't care so much about age. Embedded tends to be one (although not the Detroit car companies).
Get out of the conventional salaried programming career ("Nobody ever got rich typing"). As noted by others, you can move to management (which is genuinely challenging). You can be a consultant, although that tends to be a hard life. Or otherwise become your own boss, i.e. be a founder of startups.
Note that my experience is on the East Coast; I've heard that age discrimination in the Bay area is much worse.