My girlfriend is 21, and she has large, sagging breasts. What's her best option?

Thank for your question.  She will need to be evaluated in person by a board-certified plastic surgeon to determine what he best option is.  Typically, a breast reduction candidate is a woman whose overly-large breasts cause physical pain and mental/emotional distress.  The procedure will remove tissue as well as lift the breasts.All the best,Dr. ResultsMiami, FL

Hello dear, thanks for your post and provided pictures as well. Patients who goes to a consultation for drooping breasts (ptosis) or
volume loss after pregnancy (pseudoptosis) almost always have one very
important question. "Can my drooping be corrected with just implants or
do I need a lift".

A breast lift raises the breasts by removing excess skin and tightening the
surrounding tissue to reshape and support the new breast contour.A breast lift (with or without an implant) is better when:
Breast drooping is moderate or severeThe nipple is at the bottom of the breast or points downwardThere is no visible skin under the nipple/areola when looking at the breasts straight on.The patient is already a C or D cup breast size and doesn't want to be largerThe areola is too large and the patient wants it reduced  
And
also if you want a smaller cup I do recommend a reduction too because is
a procedure to remove excess breast fat, glandular tissue, and skin
to achieve a breast size in proportion with your body and to alleviate
the discomfort associated with overly large breasts.I recommend to see a board certified plastic surgeon to talk about your goals and for a persona evaluation. Good luck :)

Thank you for the question and your concern for your girlfriend.  That said see some experts in your area as she sounds like she would benefit from a breast reductionDr. Corbin