You probably got the "he seems smart, but I would expect someone with this much experience to gave progressed farther in his career, no hire because no trajectory , we can't hire someone with 15 years experience at the '7 years of experience' level he interviewed at". For people who are solid contributors and not "management material", this looks a lot like ageism, but technically maybe isn't.
Now, why they want everyone at a 50K engineer company to be management material...seems ill advised.
Obviously, I have no idea why I was rejected. If that is the case, no part of their interview tests what a lot of solid devs spend tons of their time doing in their career... solving problems. I have debugged countless problems. It's not sexy, not resume material, but it is reality. There are many, many very good devs working in smaller companies who have made a career out of getting things done. If Google isn't interested in people like that, I question that wisdom.
You also can't make a determination about "management material" by simply looking at a resume and seeing "they aren't a manager". Why am I not a manager, though? Because I still love development. It doesn't mean I don't mentor people. It doesn't mean I don't hold a lot of weight in determining how we do things.
I'm not really claiming ageism. I'm in my mid-30's. I'd be surprised if that was it. I do wonder if it's wise having 20-somethings interviewing people though.
Now, why they want everyone at a 50K engineer company to be management material...seems ill advised.