An adult outfield glove is 12–12.75" on Rawlings' chart and 12–13" on Baseball Monkey's: the sizer picks 12.75". Fastpitch outfielders use 12–13" and slowpitch outfielders up to 14", with an H-web or trapeze for a deep pocket that holds a fly ball at full stretch.
Two charts at 14–18
Rawlings lists adult 12+, outfield at 12–12.75"; Baseball Monkey’s baseball, ages 14 and over band gives 12–13". They overlap at 12–12.75", and the sizer picks the middle: 12.75".
At 14–18 a pitcher’s glove (12") wants a closed or basket web, a outfield glove of 12.75" an H-Web, and a third baseman’s 12" an H-web.
The adult game
High school, college and adult baseball is played on the 90-foot diamond with a 60 ft 6 in pitching distance, and gloves are adult patterns throughout.
CDC growth charts put the median 18½-year-old boy at 176.4 cm (5'9") and girl at 163.2 cm (5'4"); the 5th to 95th percentile for boys runs 5'5" to 6'2". 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 14–18 in Adult Medium (7.25–7.5") and Adult Large (7.5–7.75") and Adult XL (7.75–8") and Adult 2XL (8–8.25"), 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 12.75", a shorter one toward 12".
For comparison, the high school & adult infield sheet starts from 11.5".