If a portion of a wrist growth plate is no longer growing in 14 y.o. girl & should it be treated when discovered or wait until she is older?

Dozens of plates: You need to confer with your doc on this matter. There are dozens of growth plates in the hand & wrist that undergo sequential closure as part of normal growth.That's how we determine if a kid is progressing through a normal pattern of bone maturity. By the time they are 17-18 most have completely fused. Partial closure at 14 is likely part of that normal progression & reflects hormonal activity.