You'll hate this boomer talk, but back in the day, a lot of youngsters ran the marathon. As sophomore in college kid at my school ran 2:29 on a course tougher than Boston's. Thom Hunt did 2:17 in HS. Gompers was 2:14 as a college freshman. Kirk Pfeffer ran 2:12 at NYC as a 21 yr old. Our lousy little small college conf. had 2 OT qualifiers who ran 2:20 - 21 at 20 or 21....
All these are somewhat exceptional, but 25 years ago a lot of people made their marathon debut by the age of 24 or 25. I've always felt that Salazar changed that. He was brilliant at 22-24, but flamed out by the time he was 25. Everybody became more cautious after that.
Note: 2:32 is pretty much in line with 15:30 (but that's not to say that any and every 15:30 runner can expect to run a 2:32).