Top season by 20 year old batter.

MikeTrout?

Player ages are computed as their age on June 30th.

For single seasons, From 1903 to 2012, For age 20, (requiring onbase_plus_slugging_plus>=150 and Qualified for league batting title), sorted by greatest Adjusted OPS+

Rk	Player	OPS+	Year	Age	Tm	Lg	G	PA	AB	R	H	2B	3B	HR	RBI	BB	IBB	SO	HBP	SH	SF	GDP	SB	CS	BA	OBP	SLG	OPS	Pos
1	Mike Trout	171	2012	20	LAA	AL	139	639	559	129	182	27	8	30	83	67	4	139	6	0	7	7	49	5	.326	.399	.564	.963	*87/9
2	Ty Cobb	167	1907	20	DET	AL	150	642	605	97	212	28	14	5	119	24		54	5	8			53		.350	.380	.468	.848	*9
3	Mel Ott	165	1929	20	NYG	NL	150	675	545	138	179	37	2	42	151	113		38	6	10			6		.328	.449	.635	1.084	*9/84
4	Al Kaline	162	1955	20	DET	AL	152	681	588	121	200	24	8	27	102	82	12	57	5	0	6	13	6	8	.340	.421	.546	.967	*9
5	Mickey Mantle	162	1952	20	NYY	AL	142	626	549	94	171	37	7	23	87	75		111	0	2		5	4	1	.311	.394	.530	.924	*89/5
6	Alex Rodriguez	161	1996	20	SEA	AL	146	677	601	141	215	54	1	36	123	59	1	104	4	6	7	15	15	4	.358	.414	.631	1.045	*6
7	Ted Williams	160	1939	20	BOS	AL	149	675	565	131	185	44	11	31	145	107		64	2	3		10	2	1	.327	.436	.609	1.045	*9
8	Rogers Hornsby	150	1916	20	STL	NL	139	550	495	63	155	17	15	6	65	40		63	4	11			17	0	.313	.369	.444	.814	*563/4

Provided by Baseball-Reference.com: View Play Index Tool Used

Generated 11/15/2012.

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For single seasons, From 1903 to 2012, For age 20, (requiring WAR_bat>=5 and Qualified for league batting title), sorted by greatest WAR Position Players

Rk	Player	WAR/pos	Year	Age	Tm	Lg	G	PA	AB	R	H	2B	3B	HR	RBI	BB	IBB	SO	HBP	SH	SF	GDP	SB	CS	BA	OBP	SLG	OPS	Pos
1	Mike Trout	10.7	2012	20	LAA	AL	139	639	559	129	182	27	8	30	83	67	4	139	6	0	7	7	49	5	.326	.399	.564	.963	*87/9
2	Alex Rodriguez	9.2	1996	20	SEA	AL	146	677	601	141	215	54	1	36	123	59	1	104	4	6	7	15	15	4	.358	.414	.631	1.045	*6
3	Al Kaline	8.0	1955	20	DET	AL	152	681	588	121	200	24	8	27	102	82	12	57	5	0	6	13	6	8	.340	.421	.546	.967	*9
4	Mel Ott	7.3	1929	20	NYG	NL	150	675	545	138	179	37	2	42	151	113		38	6	10			6		.328	.449	.635	1.084	*9/84
5	Ted Williams	6.6	1939	20	BOS	AL	149	675	565	131	185	44	11	31	145	107		64	2	3		10	2	1	.327	.436	.609	1.045	*9
6	Ty Cobb	6.6	1907	20	DET	AL	150	642	605	97	212	28	14	5	119	24		54	5	8			53		.350	.380	.468	.848	*9
7	Jason Heyward	6.3	2010	20	ATL	NL	142	623	520	83	144	29	5	18	72	91	2	128	10	0	2	13	11	6	.277	.393	.456	.849	*9
8	Vada Pinson	6.3	1959	20	CIN	NL	154	706	648	131	205	47	9	20	84	55	3	98	1	2	0	9	21	6	.316	.371	.509	.880	*8
9	Mickey Mantle	6.3	1952	20	NYY	AL	142	626	549	94	171	37	7	23	87	75		111	0	2		5	4	1	.311	.394	.530	.924	*89/5
10	Frank Robinson	6.2	1956	20	CIN	NL	152	667	572	122	166	27	6	38	83	64	7	95	20	8	4	14	8	4	.290	.379	.558	.936	*7/8
11	Ken Griffey	5.0	1990	20	SEA	AL	155	666	597	91	179	28	7	22	80	63	12	81	2	0	4	12	16	11	.300	.366	.481	.847	*8/D

Provided by Baseball-Reference.com: View Play Index Tool Used

Generated 11/15/2012.

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For single seasons, From 1903 to 2012, For age 20, (requiring runs_bat>=40 and Qualified for league batting title), sorted by greatest runs_bat

Rk	Player	Rbat	Year	Age	Tm	Lg	G	PA	AB	R	H	2B	3B	HR	RBI	BB	IBB	SO	HBP	SH	SF	GDP	SB	CS	BA	OBP	SLG	OPS	Pos
1	Mel Ott	61	1929	20	NYG	NL	150	675	545	138	179	37	2	42	151	113		38	6	10			6		.328	.449	.635	1.084	*9/84
2	Alex Rodriguez	58	1996	20	SEA	AL	146	677	601	141	215	54	1	36	123	59	1	104	4	6	7	15	15	4	.358	.414	.631	1.045	*6
3	Mike Trout	54	2012	20	LAA	AL	139	639	559	129	182	27	8	30	83	67	4	139	6	0	7	7	49	5	.326	.399	.564	.963	*87/9
4	Ted Williams	54	1939	20	BOS	AL	149	675	565	131	185	44	11	31	145	107		64	2	3		10	2	1	.327	.436	.609	1.045	*9
5	Al Kaline	53	1955	20	DET	AL	152	681	588	121	200	24	8	27	102	82	12	57	5	0	6	13	6	8	.340	.421	.546	.967	*9
6	Mickey Mantle	42	1952	20	NYY	AL	142	626	549	94	171	37	7	23	87	75		111	0	2		5	4	1	.311	.394	.530	.924	*89/5

Provided by Baseball-Reference.com: View Play Index Tool Used
Generated 11/15/2012.