The idea of professional senescence beginning with the fourth decade of life seems to originate in academia, where publication of original research is the figure of merit for professional value, and does indeed appear, in the aggregate, to peak late in the third decade.
But academia is a profession more unlike than like our own. We do, after all, call ourselves not scientists but engineers. While the role of an engineer may include the invention of novel applications or even novel theory, it need not; what we do revolves instead around the application of ingenuity to the solution of real problems, and I see no reason to assume that facility of ingenuity or skill in its application are dependent upon youth. On the contrary, the accrual of experiential knowledge which comes with a long period of practice in the field makes a canny engineer more capable, rather than less so.
Not to say that there's no place for genius, of course - but, again, genius is hardly the sole province of youth. And ours is a young field, as the professions go; ours is the third or perhaps the fourth generation in all human history where the phrase "software engineer" has even had a defined meaning, and so it's reasonable to expect we may still be somewhat bound by the traditions of the theoretical fields in which our own so recently originated - we haven't really had time to establish traditions of our own, and the field is something of a free-for-all as a result. Combine all that with one of our preeminent centers of practice by happenstance being a place which has fetishized youth and beauty for well longer than our field's been around - and which has also, separately and for quite a few more years, made an industry of exporting its own culture to our country and the world - and it's not hard to see how we might end up with the kinds of attitudes here under discussion.
There is, though, cause for considerable optimism in the fact that Silicon Valley is not the reliable metonym for software engineering culture which it is so often taken to be, whether by the self-congratulatory among its own inhabitants or by those naïve or ignorant enough to believe the hype they expound. Sure - the field was in considerable part incubated there, and largely codified there to the extent it has been codified at all. But that a thing was invented in a place does not give that place an exclusive, permanent claim on defining the nature of the thing.
Sure - if you're forty or fifty and you want to work for Google as a software engineer, you're going to have a bad time, not least because you'll probably be interviewed by infants who may find you reminiscent of the fathers in their relationships with whom they are not yet fully past the adversarial phase. But there are many places to work other than Google, and many more preferable places to work and to live than Silicon Valley. There is no reason for anyone, but especially anyone who knows better, to subject herself to the programmer's equivalent of the Hollywood talent mill - often, as we have lately been hearing in the news, complete with "casting couches" for those who happen to suit the sexual fancy of one who happens to hold some power.
Come and join us out here in the real world! We'll be happy to have you among us, and you can make a life for yourself here. Believe it or not, we even have some houses that sell for under a million!