Breast Reduction for a 14 Year Old?

This is clearly a complex matter and should be handled with a multidisciplinary team involving her therapists, pediatrician, plastic surgeon, and possibly and endocrinologist.
In regards to timing, she seems very young and she may not have completed her growth. In this regard, if it is performed too early, she need to have the procedure performed again due to ongoing growth. Clearly, it would be preferable to have her growth curve plateau and then contemplate the surgery.
The red stretch marks can be treated with retin-a or laser but will do little to alleviate the size of the breasts

It sounds like you are dealing with juvenile breast hypertrophy along with the physical and psychosocial consequences of this diagnosis. In other words, the breasts are too large for the frame causing  both physical and psychological distress.

It sounds like you have done your homework and understand the potential risk and consultations associated with breast reduction surgery. You also understand that further surgery may be necessary in the future (for example if the breasts were to grow in size again).

On the other hand, breast reduction surgery is one of the most patient pleasing operations we perform and I think that for the right teenager (enough symptoms) it may be an excellent option (regardless of the age).

I would suggest an in-person consultation with a well-trained/experienced board-certified plastic surgeon. Best wishes.

Large breasts can be not only a psychological problem but a functional problem, teens included. However, operations on teenagers must be done with careful attention paid to their expectations of the operation. If those expectations are realistic, the operation will succeed. If they see the operation as a means of solving social problems, it will not and might even be a disaster.
As a parent, the responsibility for choosing an elective operation for a teenager lies with you. Take your time to make sure this is something your child will truly benefit from. Based on your description, it might be wise to get the opinion of a psychologist or psychiatrist.