They are old enough to learn well and perform at a relatively high level, but not old enoughh to know "everything," like many teenagers.
These guys already know much of the stuff on the younger lists. They need to learn new stuff and refine the stuff they already know. Plus you can teach them the following new things:
30. first-and-third defense plays
31. taking extra base on outfielder's throw to pitcher or wrong base
32. delayed-steal defense
33. defense against long secondary lead at third
34. squeeze bunt
35. "day-o" play (pick off runner at second with shortstop when daylight can be seen between runner and shortstop
36. catcher back up first when no runners
37. catcher block low pitches
38. defense against runner taking big secondary lead at second
39. pitcher-cover-home-on-passed-ball play
40. appeal half-swing to base ump
41. appeal for not tagging on fly or not touching base
42. runners going on 2-out, 3-2 count, forced-situation swing
43. runners going on 2-out, 2 strike, forced-situation swing
44. try to draw pick-off throw on secondary lead at all three bases
45. advance to next base on infielder's throw to first
46. score from third on infield ground ball
47. make sure you are out before leaving base after close play
48. base runners advance two bases on single
49. relay technique
50. base-coaching signals
51. pitching from stretch
52. fastball grip
53. changeup
54. outfield technique for different situations
Notice what is not in this list - things like batting mechanics, pitching mechanics, hitting the cut man, the two-throw double play. That stuff is either too advanced or too time-consuming for youth baseball.