Age 18 years, 48 days. Pete Richardson National High school record - 6/6/1981 1:47.31 - Amazing 3rd 200 split. He had a 47.7 400 PR
(Splits: 27.2, 26.8 [54.0], 25.8 [1:19.8], 27.5)
He never got much faster. https://www.worldathletics.org/athletes/united-states/pete-richardson-14359399
A 1% improvement over 41 years would be about 1:46.23. I would have thought we would have had a High school record holder closer to 1:44.23 by now- by a true 18 year old High School Senior. The U.S. Junior class record should be closer to 1:46.23.
Currently held by Elijah Greer 1:47.68 - he ended up with a personal best 6 years later of 1:44.91
I don't think many of these High school phenoms ever make it on the world stage. Although I guess Brazier is an exception, he wasn't a record holder in High school.
Johnny Gray never broke 1:50 in High school- his best was 1:51.1,
He didn't really have a break though until 4 years later. 1:45.41
Of course he went on to be the American Record holder and was still running 1:43.9 at age 36.