It's a personal decision. If it's for damage or a malformation, I think it is different than just doing it to "look better."
In my profession, I have worked with some women who have had breast implants. Most of them thought it would "change their entire life." When it didn't (and it never did) they got very depressed. Some of them go on to have more and more surgery to get that happiness they think is only one procedure away. It never works. I've worked with a small number of women who have had breast reductions and they all regretted it. (remember I work with breastfeeding women, and reductions seriously impact a woman's ability to make milk. Breast implants don't, but the removal of tissue and moving of the nipple with breast reduction can cause serious issues with the ability to make milk.)
I'd advise women of childbearing age to wait until they are completely done with childbearing, or know
they do not want children and never will and preferably in menopause before reducing their breasts and destroying their milk ducts in the process. I know SO many women who thought they were "done" having kids, then either got divorced and remarried or simply got pregnant, and then had terrible problems with their reductions. It's not something that can be put back the way it was, and causes permanent damage to the breast. (Unlike implants which can be removed and little harm is done to the breast.)