Robin Yount has the most career games by a player younger than 21, with 398 games.

Interpreted as:

most regular-season games before turning 21 years old by a player

	NAME	G	AB	R	H	2B	3B	HR	RBI	BB	HBP	SO	SB	CS	PA	TB	XBH	SH	SF	IBB	GIDP	AVG	OBP	SLG	OPS
1	Robin Yount R. Yount	398	1,469	171	383	59	10	13	129	83	2	178	34	21	1,590	501	82	23	13	6	25	.261	.299	.341	.640
2	Mel Ott M. Ott	391	1,203	237	388	72	9	61	253	180	8	92	12		1,416	661	142	25				.323	.414	.549	.964
3	Ed Kranepool E. Kranepool	361	1,224	113	299	56	10	22	112	89	4	172	5	8	1,336	441	88	5	14	9	33	.244	.295	.360	.655
4	Al Kaline A. Kaline	320	1,120	173	346	42	11	32	147	105	6	106	16	13	1,246	506	85	7	8	12	36	.309	.369	.452	.821
5	Phil Cavarretta P. Cavarretta	315	1,162	154	315	51	17	19	155	68	7	114	15		1,250	457	87	13				.271	.315	.393	.709
6	Freddie Lindstrom F. Lindstrom	297	977	152	286	37	22	13	113	67	5	51	19		1,085	406	72	36				.293	.341	.416	.757
7	Ken Griffey Jr. K. Griffey Jr.	282	1,052	152	299	51	7	38	141	107	4	164	32	18	1,172	478	96	1	8	20	16	.284	.350	.454	.805
8	Juan Soto J. Soto	266	956	187	274	57	6	56	180	187	3	231	17	3	1,153	511	119	1	6	13	20	.287	.403	.535	.937
9	Sibby Sisti S. Sisti	261	959	130	242	39	8	8	76	70	5	140	12		1,049	321	55	15			12	.252	.307	.335	.641
10	Bryce Harper B. Harper	257	957	169	260	50	12	42	117	117	7	214	29	10	1,094	460	104	6	7	4	12	.272	.353	.481	.834
11	Cesar Cedeno C. Cedeno	251	966	131	271	61	10	17	123	40	5	159	37	13	1,026	403	88	6	9	7	17	.281	.310	.417	.727
12	Travis Jackson T. Jackson	250	932	127	270	38	15	15	113	42	0	98	9		992	383	68	18				.290	.320	.411	.731
13	Tony Conigliaro T. Conigliaro	249	925	151	257	42	7	56	134	85	10	194	6	6	1,029	481	105	6	3	7	20	.278	.344	.520	.864
14	Cass Michaels C. Michaels	249	811	88	196	20	6	3	81	68	7	69	17		907	237	29	21			19	.242	.306	.292	.598
15	Buddy Lewis B. Lewis	245	1,042	172	308	43	17	11	117	77	5	79	10		1,136	418	71	12				.296	.347	.401	.748
16	Rusty Staub R. Staub	239	805	69	178	27	6	14	80	80	8	89	1	1	905	259	47	6	6	12	18	.221	.296	.322	.618
17	Mickey Mantle M. Mantle	238	890	155	262	48	12	36	152	118	0	185	12	8	1,012	442	96	4			8	.294	.377	.497	.874
18	Adrian Beltre A. Beltre	233	750	105	199	38	5	23	91	75	9	143	21	9	845	316	66	6	5	12	9	.265	.337	.421	.759
19	Jimmie Foxx J. Foxx	215	571	118	189	38	16	16	105	76	2	61	6		664	307	70	15				.331	.411	.538	.949
20	Ivan Rodriguez I. Rodriguez	211	700	63	183	32	1	11	64	29	1	115	0	1	742	250	44	9	3	2	25	.261	.291	.357	.648
21	Bill Mazeroski B. Mazeroski	208	702	83	192	31	6	11	62	44	1	67	2	3	762	268	48	13	2	3	10	.274	.316	.382	.698
22	Bob Kennedy B. Kennedy	208	788	83	186	28	6	3	71	53	0	74	6		855	235	37	14			21	.236	.284	.298	.582
23	Andruw Jones A. Jones	203	571	82	127	28	3	26	93	68	5	152	25	11	652	239	57	5	3	2	12	.222	.309	.419	.728
24	Bobby Doerr B. Doerr	200	656	92	180	31	8	7	94	76	1	64	7		759	248	46	26				.274	.351	.378	.729
25	Claudell Washington C. Washington	195	704	83	212	31	9	8	83	38	5	104	44	21	757	285	48	2	8	7	16	.301	.338	.405	.743