I'm a 21 year old with size 34D breasts that sag and I was wondering whether I should consider a breast lift?

You certainly sound like a good candidate for a breast lift - which would likely use an anchor type incision.  That being said, timing of surgery has to do with a number of factors - really too many to list... Best to consult with a board certified plastic surgeon.

It is my pleasure to respond, in order for a recommendation i need to evaluate you photos for a profesional answer. 



I advice to post pictures to get a better evaluation
and recommendations. Find a good Board Certified Plastic & Reconstructive
surgeon.



Dr. Manuel Diaz

To elevate the nipple and breast tissue it will be necessary to have a circumferential scar around the areola and a vertical scar from the areola to the breast fold.  In most cases the scars are good and with time fade to faint white lines (depends on skin type),  however, will always be present.  Perhaps pre and post operative photos or someone you know who has had the procedure to show you the improvement in shape and what the scars could look like may be beneficial.  It is understandable that you are undecided when you do not know what to expect.. You must be sure before you proceed.