Or maybe I'm just not smart enough to hang out on Hacker News.
Kylian Mbappe won the World Cup a couple of weeks ago. He was 19. That makes it a bigger deal.
Generally people are impressed by accomplishments made by younger people. I suppose there is also a similar effect at the other end, for a similar reason: you're disadvantaged in accomplishing stuff if you're at either end of the age spectrum. You either had a lot less time to get to the top, or your faculties are waning precipitously.
Not saying this applies to Tang. But would you be equally impressed by a 17 year old from an upper middle class family whose parents recognized their gifts and used their influence to assist them in skipping 3 grades and enrolling in college early vs 17 year old from lower income family who is equally bright as the former kid, but graduates at 21 and receives no recognition for it due to an unheard of advisor at a low ranked school? On paper you'd be more amazed by the former, while the latter is equally as bright but is sort of dismissed because of their situation.