Some IC roles do fit more younger candidates than older. E.g. in a startup - often better they not have outside commitments like a family, or need for a healthy work-life balance. Sometimes "work smarter not harder" falls apart and they need to work like a maniac. On balance I've found older colleagues less willing to do that. That isn't meant as a criticism, just an observation about role-fit, those people usually didn't last long.
That's not the underlying belief in a startup. It's more like: we have not fully figured out market fit. We may need to build something very very quickly (I'm talking about hours/days, not weeks/months) and then maintain them for while.
That's why I say "work smarter not harder" can fail - that only works if you make assumptions about the future, in a startup that can only take you so far.
