I can’t argue definitively as I know just about nothing about postal service personnel operations. I do know something doesn’t seem correct tho. I carried on a fairly long conversation with the young postal employee and am sure he would have mentioned his being a disabled veteran as we discussed salaries at length.
I used to have a substitute carrier who didn’t look a day over 16 tho I guess she would have had to be at least 18. She was about as pretty as a girl gets. I wondered how such a young girl got the job. Probably just showed up and smiled sweetly at someone.
It's possible your young friend was faking it, or maybe he was sharing a job. We had that problem years back in the San Francisco area. You'd get half a dozen hippies doing that.
It wasn't lawful. Postal officials would fire them when they'd catch them.