I am 20 years old with deep/dark smile lines. What can I do to get rid of this and have a more youthful looking face for my age?

Thank you for sharing your question and photographs.  Though your full face is not included in your pictures to offer the best advice, you can smooth the transition from your cheek to your lip region with carefully placed hyaluronic acid dermal fillers like Juvederm Ultra Plus or Vollure.  Be sure to see an ASPS board certified plastic surgeon in consultation to discuss. 

Nasolabial Folds that are Apparent at an early age like you're seeing in yourself, are usually seen in patients who have very shallow cheek bone and are lacking fat presence in the upper portion of the face. If this is the case, this can be remedied with fat grafting which will provide you with a natural and long lasting results. On the other hand if I am incorrect in my assumption, you can easily receive a treatment using Restylane or Juvederm to fill those lines out and I'm sure you will be satisfied in the future. Good luck from New York City