1) "Novice" (and "Junior" and "Senior") in the US are test levels, with no age limits either upper or lower. Remember, we currently have a US senior ladies' champion who was doing triple axels at 13 (but too young for junior internationals) and quads at 14.
2) Beginning last year, the top 2 novices from each section are invited to compete as juniors at Nationals.
It is true that the novices who took advantage of this opportunity tended to be younger than the national qualifiers who competed junior at sectionals (or who had JGP byes), but there is no strict age division between the two groups. However, once a skater passes the Junior freeskating test, s/he is no longer permitted to compete at novice level within the US.
Last season, skaters might have chosen to compete novice at sectionals because they thought they would have an easier path to Nationals by that route in their particular section that year, or because they had automatically qualified for sectionals at the novice level in via the NQS and had not done so at junior level. (Some skaters competed at both levels during the summer and chose which level to register for the qualifying season based on at which level they earned the NQS bye to sectionals.)
But for those who did qualify for Nationals, they would be competing against full-time juniors there, so they would adapt their Nationals programs to junior requirements, including skills allowed at that level.
3) There are very fewer skaters who might have competed Intermediate or Novice the season before, now deciding whether to compete Novice or irrevocably test up to Junior, who are seriously working on quads. In most cases they would already be over 13, but, especially among the boys, not yet physically strong enough to be competitive against 18-year-old junior men.
If the quad wasn't competition ready in August, they would have less reason to test up to junior and compete junior at regionals/sectionals. But if the quad was ready by Sectionals in November, it would help them make their case against the other novices for one of those top two spots advancing them to Nationals in Juniors.
My understand is that it was primarily with these especially talented early-teen boys in mind that the rule was proposed. Although there are now some girls of similar age also dabbling in quads and also perhaps strategizing whether it will be easier to get to Nationals by competing novice in the summer/fall.
Much of the above is moot for the 2020-21 season given the health crisis-related rule changes and relaxations for this season. E.g., there was no NQS this year and there will be no regionals with advancement to sectionals -- there will only be one level of competition offered in November-December from which skaters will advance (though we don't yet know the details). And it will be legal this year for skaters who have not passed even the novice test to compete as juniors. But those changes had not yet been implemented when the rule was first proposed or even when the vote took place at Governing Council.
