TLDR: I'm a 40-year-old developer. It took me a little longer to find a job at 40 than 30 because I demand a higher salary, and most companies prefer cheap over experienced. There are a lot of mediocre developers who I suspect "age out" of tech because they can't grow into industry expectations of a highly-experienced software developer.
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I searched for a developer job just before I turned 30, stayed with it for 10 years, and then searched for another when I turned 40.
My experience of ageism is that most companies hiring just don't have the budget to hire an experienced software developer. They'd rather hire a few more cheap (and inexperienced) developers than hire a few highly experienced, and expensive, software developers.
To me, this translated in a longer job search. Not unreasonably longer, as it took me about a month to find a job when I was almost 30, and three months right before I turned 40. (Worth noting, when I was 30 I lived in Silicon Valley, and it's much easier to find a developer job there compared to where I live now.)
But, I do want to point out something that most articles of ageism don't point out: Expectations of a 40+ year old developer are quite different compared to a 20-30 year old developer. The kind of handholding that a junior developer has, and mentorship that a junior developer has, is gone. It's also much harder to change stacks than when I was a junior developer, because no one wants to pay me big bucks to learn like a novice. Thus, it's hard to change stacks. (Not impossible, but I really like the job I have now.)
Another thing to point out: There are a lot of mediocre developers who get very comfortable with handholding in the early part of their career. I suspect these are the developers who really struggle in their 40s, because they can't meet the expectations of a senior developer.