Youngest GMs

According to this Chess.com video, the earliest of the youngest GMs -- starting with Botvinnik & Reshevsky -- were in the original group of GMs nominated by FIDE (1950). The first GM under-20 was Boris Spassky (1955), followed by Bobby Fischer (1958). Fischer's record was finally bested by Judit Polgar (1992), after which a new 'youngest' becomes a more frequent occurrence.

The Youngest Chess Grandmasters Of All Time (2:11) • '[Published on] Feb 27, 2020'

The first comment points to The Youngest Chess Grandmasters In History (chess.com; Published: Mar 22, 2019, Updated: Feb 27, 2020). For a similar video, also from Chess.com, see The Most Followed Chess Streamers Over Time.