What else was the agent supposed to do? Guy wasn't pre-check.
If your argument is that in the bigger picture, we should *all* be proceeding through a checkpoint that looks like the pre-check process, because the other stuff is just security theatre anyway, then you won't get much disagreement from me.
But with today's rules, and for that front-line agent, that's probably correct protocol. Politely redirect the non-pre-check guy over to the nude-o-scopes and shoe fetishists.
I don't see how age matters, beyond the fact that he gets to keep his shoes on in either line.
Also, the TSA's job isn't to keep people happy. If that were their job, they would have to start by disbanding.