Dwight Howard has grabbed the most career rebounds by a player younger than 21, with 2,109 boards.

nba most rebounds by nba player under age 21

NAME	REB	GP	MPG	PPG	RPG	APG	SPG	BPG	TPG	FGM	FGA	FG%	3PM	3PA	3P%	FTM	FTA	FT%	MIN	PTS	AST	STL	BLK	TOV	PF	+/-
Dwight Howard	2,109	184	34.9	14.2	11.5	1.3	0.8	1.6	2.4	5.1	9.5	53.5	0.0	0.0	0.0	4.0	6.4	62.9	6,415	2,619	233	152	292	449	562	-106
Andre Drummond	1,528	141	27.4	11.1	10.8	0.5	1.1	1.6	1.2	4.9	7.9	61.8	0.0	0.0	25.0	1.4	3.5	40.2	3,862	1,571	65	160	226	167	417	-284
Jalen Duren	1,304	128	26.9	11.4	10.2	1.7	0.6	0.8	1.7	4.7	7.5	63.1	0.0	0.0	0.0	1.9	2.6	70.7	3,448	1,453	222	77	106	221	374	-590
LeBron James	1,187	186	41.0	25.0	6.4	6.4	1.9	0.7	3.4	9.1	20.1	45.4	1.1	3.5	33.0	5.6	7.4	75.5	7,629	4,649	1,193	352	135	626	361	+165
Jabari Smith Jr.	1,185	155	31.4	13.3	7.6	1.4	0.6	0.9	1.2	4.8	11.1	43.0	1.7	5.0	33.5	2.0	2.5	79.8	4,874	2,054	222	95	135	193	427	-638
Chris Bosh	1,124	141	34.8	13.7	8.0	1.4	0.9	1.4	1.8	5.0	10.6	46.5	0.0	0.1	33.3	3.7	5.2	72.2	4,901	1,930	203	122	194	254	401	-140
Andris Biedrins	1,122	172	21.0	6.5	6.5	0.7	0.6	1.2	0.9	2.9	4.8	60.8	0.0	0.0	0.0	0.7	1.6	46.0	3,606	1,122	120	97	199	159	551	+131
Kevin Garnett	1,119	157	33.7	13.7	7.1	2.4	1.2	1.9	1.8	5.8	11.7	49.6	0.1	0.3	28.6	2.0	2.7	73.6	5,288	2,144	381	191	294	285	388	+116
Josh Smith	1,110	169	30.5	10.6	6.6	2.1	0.8	2.3	2.0	3.9	9.1	43.2	0.3	1.1	27.1	2.5	3.5	70.3	5,159	1,799	363	142	381	338	467	-697
Anthony Davis	1,084	119	31.9	16.8	9.1	1.2	1.3	2.3	1.5	6.5	12.6	51.9	0.0	0.1	15.4	3.7	4.9	76.2	3,802	2,001	148	153	272	184	330	-242
Alperen Sengun	1,071	147	24.9	12.3	7.3	3.2	0.9	0.9	2.3	4.7	9.0	52.2	0.3	1.2	27.6	2.5	3.5	71.3	3,660	1,801	476	129	138	338	472	-592
Isaiah Stewart	1,070	139	23.5	8.1	7.7	1.0	0.4	1.2	1.1	3.4	6.5	52.9	0.3	0.8	33.0	1.0	1.4	70.8	3,270	1,126	141	61	164	152	395	-427
Jarrett Allen	1,060	152	23.3	9.6	7.0	1.0	0.5	1.4	1.2	3.7	6.3	59.0	0.1	0.4	18.3	2.0	2.8	73.2	3,537	1,460	159	71	208	185	331	-264
Tracy McGrady	1,048	192	24.7	11.1	5.5	2.5	1.0	1.4	1.6	4.2	9.4	44.8	0.2	0.7	28.4	2.4	3.4	71.2	4,747	2,122	474	191	278	306	381	-262
Josh Giddey	1,020	130	31.3	14.9	7.8	6.3	0.8	0.4	2.9	6.3	13.8	45.9	1.0	3.4	29.5	1.2	1.7	72.3	4,068	1,934	814	108	51	382	231	-404
Luka Doncic	1,012	119	32.4	24.2	8.5	7.1	1.1	0.3	3.7	8.0	18.0	44.3	2.6	7.9	32.5	5.6	7.6	73.6	3,857	2,876	839	126	34	446	253	+142
Giannis Antetokounmpo	1,006	178	28.6	10.5	5.7	2.3	0.8	0.9	1.9	3.8	8.1	47.1	0.3	1.1	29.7	2.6	3.6	73.7	5,095	1,874	410	151	169	342	502	-254
Bismack Biyombo	950	143	25.5	5.0	6.6	0.4	0.3	1.8	1.1	2.0	4.4	45.7	0.0	0.0		1.0	2.0	50.2	3,641	713	62	48	258	153	340	-833
Karl-Anthony Towns	939	91	32.3	18.7	10.3	2.0	0.7	1.6	2.2	7.7	14.3	53.7	0.5	1.3	35.8	2.8	3.5	80.3	2,943	1,702	185	66	149	202	275	-127
Darius Miles	930	163	26.7	9.4	5.7	1.7	0.7	1.4	1.9	3.8	7.8	49.3	0.0	0.2	10.5	1.7	3.0	57.2	4,360	1,540	283	122	228	307	375	+36
Carmelo Anthony	924	157	35.7	20.9	5.9	2.7	1.1	0.5	3.0	7.4	17.2	42.8	0.7	2.4	29.8	5.5	7.0	78.7	5,603	3,283	421	165	71	471	454	+231
Andrew Bynum	920	163	19.3	7.2	5.6	1.0	0.2	1.4	1.1	2.9	5.0	57.1	0.0	0.0	0.0	1.4	2.2	64.9	3,140	1,171	164	28	222	184	403	+80
Eddie Griffin	877	150	25.2	8.7	5.8	0.9	0.5	1.6	0.8	3.4	9.0	38.3	1.0	3.1	33.1	0.8	1.2	67.8	3,786	1,306	139	69	245	123	253	-10
Tyson Chandler	857	146	22.0	7.7	5.9	0.9	0.4	1.4	1.6	2.8	5.4	51.8	0.0	0.0		2.1	3.5	60.6	3,216	1,127	130	65	199	234	399	-513
Derrick Favors	837	143	20.4	7.7	5.9	0.6	0.5	0.9	1.3	3.1	6.0	50.8	0.0	0.0		1.6	2.6	62.4	2,911	1,103	82	67	133	180	391	-255