It was either:
A. This guy is worth twice our maximum budget
B. He's used to technical stacks that have absolutely nothing to do with ours
C. New grads perform better at leetcode than him(not saying that's a good metric, but that's not ageism)
D. Culture fit.
This is the biggest place where all kinds of biases slip in, including ageism. Culture is tied up in all of the protected classes, and any time that you hear "culture fit" used as a justification for turning someone down alarm bells should be going off.
The legitimate uses of "culture fit" as a filter should be itemized rather than lumped together: "he doesn't deal well with ambiguity", "she doesn't like frequent interruptions", whatever. Just talking about "culture fit" in the abstract is a lawsuit waiting to happen.
It often can be, as:
1. Younger folks have more recently done their degree and algorithms stuff 2. There tends to be more free time to grind leetcode at a younger age (family, responsibilities, lower energy can hamstring an older person here)
Ymmv, but I wouldn't say LC is neutral in this regard.