So I went to the ISU.org website to see about this, and found this file:
https://isu.org/figure-skating/rules/fsk-regulations-rules/file it talks about scoring PCS on page 82, and the criteria for them. I don't see anything about it been relative to other skaters, it looks like it's supposed to be scored against a mythical skater, who can do all the stated criteria perfectly.
So I guess people are right in saying Plushy is incorrect to say a 14 year old can't receive 10s purely because they're 14, but it's not wrong to say that 10s should be given out very sparingly. When you give a skater a 10 you're basically saying they've reached perfection. This perhaps wouldn't be an issue if judges always marked to the criteria, but it's hard to imagine that a judge who gave a skater a 10 in a category would ever give them less than 10 next time unless they really messed up their program, even if at that particular event they didn't skate a 10 but a 9.5. This could impact the scoring and be the difference between a skater coming first or second or ending up off the podium, not such a big deal at a national competition, but at the Olympics or Worlds it would really mean something.