There are a few things in the face that (to me) make it seem more youthful than a 19-20 year old. So I took the liberty of playing with the first screen cap a bit.

From top to bottom ....

• The eyebrows generally follow the curve of the eye fold. So I tweaked her left brow to illustrate this a little more.

• Her cheeks might be a bit too rounded, making her appear younger. I raised her cheekbones a bit (you can see it more on HER right side of face)

• The nose is rather petite, which also makes her look younger. I added a little bit of meat to her nose tip to give it slightly more character.

• Finally I moved her jawline in some and raised her chin, which ages her just a tad as well.

These changes don't have to be major, but those are the areas that cried "youth" to me. The chin could be a little more pronounced in my version, but hopefully this gives you some ideas of areas that might help a bit.

That ... and her chest shape. It doesn't have to be a HUGE change, but the shape of her chest is more akin to a girl that is at age of puberty. I was a walking ironing board when I was in my early 20s, but I think I had more than she has. So yeah, add a little more meat there as well.