This is a sensitive topic, so I hope my question comes off as genuinely curious and not invalidating to the OPs experience, but rather a general question about how applicant selection ought to work. I wonder if the OP has some qualities that are weakly or strongly correlated with age (e.g. number of "older silicon valley institutions" one has worked at), and YC was selecting on those factors rather than explicitly filtering based on age.
My question is: if there exists some correlation between a protected class and some other variable X, is it moral to select based on X?
What happens if X is highly predictive of some conventional success metric (e.g. ability to raise a series A)?
Things that come to mind: redlining, use of zip code in credit score prediction, etc. If we deem these immoral, is the implication that we should perform selection within protected classes, and within non-protected classes (e.g. p(X|is protected class)), and guarantee equality of outcome (or non-discrimination of protected classes based on X)?
Note that I'm not saying YC was doing this - might just be standard unconscious bias.