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.
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.
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.