21 years old- After loosing weight, I have saggy smile lines and under eyes. How can I treat this? (Photos)

Please take my advice seriously: do not get a midface facelift under any circumstances in your age and with your situation, you'd end money ripped-off and with bad deformities.

Note: research about vision loss risks of fillers (of any kind, even fat) injected in the periorbital and perinasal area.

Thank you for your question. Based on your photo and description, your concerns seem to be from both volume loss and some skin laxity after weight loss. Fillers or fat grafting can help restore volume, but since you also have loose under-eye skin, they may not fully correct the issue and could cause puffiness.
A midface lift or lower blepharoplasty (with or without fat repositioning) would more directly address the sagging and hollowness by lifting the cheek area and tightening the under-eye skin. I recommend consulting a board-certified plastic or oculoplastic surgeon to assess which option is best for your anatomy.