Since 2000-01, Luka Doncic has the most triple-doubles by a player 19 or younger, with 4 triple-doubles.

most triple doubles by age 19 or less after 2000
Interpreted as:
most regular-season triple doubles by a player by age 19 or less after 2000
Luka DoncicLuka Doncic57432.,8371,1944153245716194107-16
LaMelo BallLaMelo Ball51128.815.,4698033023138118145136-84
Markelle FultzMarkelle Fultz14118.
StatMuse has game-level data for triple-doubles going back to the 1950-51 season.