Isiah Thomas had the most assists per game by a player 23 or younger when playing 20+ games, with 10.9 per game.

most assists per game by a player 23 years old or younger minimum 20 games
ThomasJohnsonPaulJacksonJohnsonStoudamireYoungTinsleyHardawayKiddMcMillanWallMarburyWilliamsRubioSimmonsKnightDoncicDouglasMorantSmithAndersonBibbyGrantVan Exel10.910.59.99.89.49.08.98.88.78.78.48.38.28.18.18.08.07.77.67.37.37.17.17.17.0
NAMEGPAPGMPGPPGRPGSPGBPGTPGFGMFGAFG%3PM3PA3P%FTMFTAFT%MINPTSREBASTSTLBLKTOVPF+/-
Isiah ThomasIsiah Thomas24410.937.721.84.28.418.046.40.44.76.375.09,1895,3221,0162,671929
Magic JohnsonMagic Johnson7910.536.816.88.66.511.854.80.00.30.03.84.880.02,9071,326683829200
Chris PaulChris Paul3009.937.219.44.82.40.12.66.814.547.00.92.534.44.95.884.911,1695,8271,4332,96672623776769+603
Mark JacksonMark Jackson1449.837.315.04.82.30.13.15.913.244.60.72.329.22.63.574.85,3722,1596931,41232912453389
Kevin JohnsonKevin Johnson2089.432.616.33.41.40.33.05.811.849.30.10.316.94.65.485.66,7803,3847071,94829859623480
Damon StoudamireDamon Stoudamire1519.040.919.64.01.50.23.76.916.841.22.05.537.13.74.681.26,1762,9656111,36222132555328-213
Trae YoungTrae Young2178.933.224.13.90.90.24.27.717.943.12.47.034.46.37.386.37,1975,2398451,92720137907375-334
Jamaal TinsleyJamaal Tinsley548.830.69.03.81.70.63.53.59.636.10.41.821.41.72.472.71,6534882054739230188176+41
Tim HardawayTim Hardaway798.733.714.73.92.10.23.35.912.547.10.31.127.42.73.576.42,6631,16231068916512260232
Jason KiddJason Kidd2018.735.313.45.82.00.33.44.812.638.51.23.832.32.53.669.37,0922,6951,1561,74241164690388+79
Nate McMillanNate McMillan1538.428.96.54.41.90.62.22.55.247.40.10.228.11.52.367.14,4259976691,28529491344476
John WallJohn Wall2668.336.017.74.31.60.63.66.314.842.60.61.930.84.45.679.09,5744,6961,1462,198430169968648-456
Stephon MarburyStephon Marbury3238.237.619.92.91.30.23.27.016.342.71.33.932.84.76.177.212,1336,4429462,641404521,040820-242
Deron WilliamsDeron Williams2428.134.415.32.90.90.22.85.812.446.61.12.837.42.73.577.48,3153,7067041,96622853670687+465
Ricky RubioRicky Rubio1808.131.810.14.12.30.12.93.28.736.80.61.832.33.13.980.15,7321,8257421,45841924524458+347
Ben SimmonsBen Simmons2148.034.416.48.31.70.83.56.812.056.10.00.18.72.94.959.47,3633,5181,7781,713367163744593+580
Brevin KnightBrevin Knight1238.030.89.43.32.20.22.63.47.844.30.00.17.12.53.378.33,7921,16240098227126315401+314
Luka DoncicLuka Doncic2117.733.425.78.41.00.44.08.719.245.62.78.133.05.57.573.37,0425,4161,7631,62621882838468+209
Sherman DouglasSherman Douglas817.630.514.32.51.80.13.05.711.649.40.10.416.12.84.068.72,4701,15520661914510246187
Ja MorantJa Morant1437.332.119.14.21.00.23.37.015.146.61.13.432.24.05.375.34,5842,7375951,04913734473215+28
Kenny SmithKenny Smith1197.337.115.62.51.40.13.06.112.947.40.41.134.13.03.977.94,4191,85829886616812353268
Kenny AndersonKenny Anderson2017.131.014.53.41.60.22.65.613.541.80.20.829.43.03.779.56,2312,9156751,43632135516409
Mike BibbyMike Bibby2947.136.614.53.31.40.22.65.712.944.70.92.435.42.12.777.510,7624,2519682,07840447775574-728
Gary GrantGary Grant697.126.911.83.32.00.13.65.111.743.30.10.321.11.72.373.71,8558172294871409250167
Nick Van ExelNick Van Exel1747.035.115.12.81.10.12.35.413.440.41.95.534.42.43.078.56,1062,6254851,22219817406336
StatMuse has game-level data for assists per game going back to the 1982-83 season.