What age do you need to be for a boob reduction to be done?

After full growth: Breast reduction surgery should be delayed till your growth is complete. If surgery is done and you are not finished growing your breasts may again grow to be too large. A second surgery would be required to further reduce the breasts.

Breastgrowth/feeding: Breast reduction surgery is usually only considered after pubertal growth has occurred but more importantly your surgeon will only consider the surgery when a woman has completed her family or is planning not to breast feed her children. Some women undergo breast reduction surgery even in their 60s.

After adolescence: your breasts will need to be fully developed before you can have a reduction. If you are planning to have children or nurse, it's better to postpone your surgery. This being said, I would do a reduction on a teenager if benefits outweigh the risk and social issues.