I'm 26 yrs old female. I have a deep nasolabial fold line due to a receding chin and down-turned mouth. Any suggestions?

It sounds like you might have a skeletal issue. I think you shoukd consult an oral and maxillofacial surgeon to get their opinion 

If you are not considering orthodontia for your teeth and possible oral surgery then a chin implant would be a simple way to correct your problem and if placed via an incision inside the mouth avoid concerns regarding scarring.

Dr. Corbin

 The curved line around your chin pad might best be treated with fillers as can the corners of the mouth. 

I believe the easiest thing, would be a small anatomical chin implant.  It will add a bit of projection to the chin to better balance the lower half of your face.  In addition to the chin, you will need fillers to add volume to the marionette lines.

I'm not sure I agree with a chin implant, at least at first. You really don't have much chin recession from the side picture...even though it's turned a little. The problem is the soft tissue around the mouth.   it's very full around the mouth and cheeks and less so around the chin.  A chin implant only amplifies part of the soft tissue and I have seen implants make people with your configuration  worse not better. Since you are young I would do some thoughtful injections, to fill in the jawline and blunt the soft tissue indentations around the chin pad. This will let you know what can be done without burning any bridges and surgery.     Val Lambros Newport Beach