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
DoncicBallFultz411
NAMEGPTD3MPGPPGRPGAPGSPGBPGTPGFGMFGAFG%3PM3PA3P%FTMFTAFT%MINPTSREBASTSTLBLKTOVPF+/-
Luka DoncicLuka Doncic57432.220.97.35.71.00.33.46.915.943.42.57.035.34.76.572.61,8371,1944153245716194107-16
LaMelo BallLaMelo Ball51128.815.75.96.11.60.42.85.713.243.61.85.135.22.53.275.81,4698033023138118145136-84
Markelle FultzMarkelle Fultz14118.17.13.13.80.90.31.23.27.940.50.00.10.00.71.547.625310043531241722+18
StatMuse has game-level data for triple-doubles going back to the 1950-51 season.