Breast Lift or Breast Reduction Surgery for 21 Year Old?

A breast reduction and a breast lift are the same except with a reduction you are making the breasts smaller. Having children may affect your surgery long term but this is a decision you and your PS should make. You should be able to breast feed but no guarantee. 

Yes, your description makes me think you would likely benefit from a breast lift/reduction.  A reduction is also a lift.  It turns out that a lift is also a reduction (it reduces the lower pole of your breasts).  They are really the same operation.  In a breast "reduction" we usually just take out more breast tissue.  The optimal technique is the vertical technique.  The inverted-T technique produces an unflattering boxy shape and excessive scars, although it is still in use.  The "donut" technique that removes tissue around the areola is not very effective in treating sagging. 

 I would recommend that you look at photos of results from different surgeons before proceeding.  There is quite a bit of variation in results, less than for breast augmentation, and you will see fewer examples of breast lifts posted online than augmentations.  If you are satisfied with the results you see, that is a good sign and you may feel more confident about the surgeon you select.  I've attached the relevant section of my own website.