A 12-year-old takes about 11.5" at second base or shortstop (11–11.5"), 11.5" at third and 12.25" in the outfield. At 12 a player can be in Junior League (60.5 ft pitching, 90 ft bases). Typical hand: 7–7.5"; median height 5'0".
Two charts at 12
Rawlings lists youth 7–12, infield at 10.25–11.5"; Baseball Monkey’s baseball, ages 11–13 band gives 11–11.5". They overlap at 11–11.5", and the sizer picks the middle: 11.5".
At 12 a pitcher’s glove (11.5") wants a closed or basket web, a middle infield (2b / ss) glove of 11.5" an I-Web, and a third baseman’s 11.5" an H-web.
The game at 12
In Little League a 12-year-old plays in Minor League (ages 5–12, 46 ft pitching, 60 ft bases); Little League (Majors) (ages 9–12, 46 ft pitching, 60 ft bases); Intermediate 50/70 (ages 11–13, 50 ft pitching, 70 ft bases); Junior League (ages 12–14, 60.5 ft pitching, 90 ft bases). Pitchers are limited to 85 pitches a day at this league age. The next step is Senior League from 13.
CDC growth charts put the median 12½-year-old boy at 152.7 cm (5'0") and girl at 154.8 cm (5'1"); the 5th to 95th percentile for boys runs 4'7" to 5'5". A tall player at the age is often a size ahead in the hand too.
Check the hand
Rawlings’ batting-glove chart puts hands at 12 in Adult Small (7–7.25") and Adult Medium (7.25–7.5"), measured from the base of the palm to the tip of the middle finger. Measure it 1:1 in Mode 01 (finger plus palm): a longer hand moves the pick toward 11.5", a shorter one toward 11".
For comparison, the 11-year-old sheet starts from 11.5".