I once taught several 70+ year old engineers SQL and they picked it up right away; they were productive in well under an hour. So, the reality is that ability to do STEM work does not evaporate at age 35. The problem is that the employment opportunities evaporate noticeably starting at age 35.

http://www.aea.org/News/nsf_numbers.htm
"[During] the early 1970s... between 60K and 100K engineers and scientists were unemployed... Remember the early 1980s when the universities were lobbying for money to expand our engineering schools, turning away domestic students and at the same time were recruiting over-seas for students?... High school students were enticed to enroll in engineering only to find they were unable to get jobs upon graduation, older engineers were laid off and salaries failed to keep up with inflation."

Edith Holleman
"When I came to the Science Committee in July of 1991 as an investigator, by the end of my first week, I had heard that the much-repeated statement from the National Science Foundation [NSF] about a pending shortage of scientists and engineers was false. When I started to ask more questions, the first people I heard from were engineers telling me it certainly wasn't true for them. Older engineers were being off-loaded into contract positions [bodyshopped], losing their benefits and their careers as quickly as the new ones were graduated. Young engineers were doing work that in years past was done by drafts-people."

"'The industry says it wants the most recent skills, the hot skills, Java, for example.', said Bard-Alan Finlan, 43, who works as a temporary senior technician. 'But I could learn Java within a month. I've sent out 200 resumes over the past 15 months, but I can't find a full-time job.'... His annual salary? $36K."

"'I'd love to have somebody with 20 years of experience, but unfortunately I'm only paying for 3 or 4.', says the IT director at a large law firm on the West Coast."

"Career coaches acknowledge that looking younger -- or looking less old -- can shorten a job search..."

Well, there are a lot more out there, including articles which mention people who did re-tool in the latest publicized buzz-words, did well in the classes, had related experience, but still can't get the time of day from recruiters. Simply being savvy and knowledgeable isn't enough... because there is a ready supply of cheap, pliant labor from over-seas, with flexible ethics such that they are willing to do basically anything.

When one is young and starting out, of course, you don't necessarily know how you measure up in professional environment, so you tend to be willing to work very hard in hopes that the quality and amount of your productivity will eventually be rewarded. After you get a few attaboys, maybe a transfer/promotion of two and things are looking up; you start thinking about maybe being able to afford to start buying a home and maybe a family. And right about then the rug gets pulled you. You may still be learning, may continue to build up your personal knowledge-base, and may frequently be consulted within the firm and by customers for your expertise, but you're considered too old, not flexible/pliant enough, possibly a threat to management, and/or potentially too expensive, and the work is let out to the bodyshoppers. This is the pattern I read in other people's personal stories and in the published articles.