I was rejected for a job when I was aged 39 for supposedly being over qualified. When I applied for the job in question I was upfront about my experience, qualifications and desired salary which was market rate for my experience. I didn't sell myself short. The product I would be working on was unified message distribution system not too dissimilar to Twilio.
After a code test, a whiteboard exam and two rounds of interviews I was told "Yeah, we really like you and you undoubtedly have the skills. But we were really looking for someone a little less qualified". I pushed a bit to find out what that meant and the HR person claimed it was my salary expectations, but I had my doubts.
During both interviews I was asked if I had children. I said I had two. I was then asked their ages and where they went to school. This led on to questions about how they got to school and what happens if one or both of them are sick etc. The questioning style was casual, but they were definitely probing me. In Ireland this line of questioning is illegal, but it happens a lot. At the time I answered the questions with a feeling of reluctance. I do believe that having young children lost me the job. I was asked what would happen if my children were sick off school, I answered that either me or my wife would have to stay home. I was asked why my wife wouldn't take on that responsibility exclusively. I responded because we share our responsibilities to our family. I could see the interviewers brow furrow when I said that.
I have a strong suspicion that the reason why age affects IT recruitment so much is not because of "over qualification". It's because the older you are the more likely you are to have family that you want to spend time with. Or if you don't have children you might have other interests that take priority over work. I've had managers who have straight up admitted that the older you are the less likely you are to put up with overtime and more likely to challenge over bearing bosses.
Put simply younger people have fewer adult life distractions, will work longer hours and put up with more crap.