You'll grow to have a gradiented forehead? That's terrible.
Just kidding. But nonetheless, fix that shading and think about it like part of a sphere.
The mouth is nice for a line, but you could add some shadows a pixel or two above and below it to suggest lips.
The nose is a bit wide, maybe move the sides in 1 pixel each. Also antialias the shadows of the sides to cure their jaggies.
The face overall is a bit misshapen. I'm really pretty horrible with anatomy, but it might be that the forehead is so short and flat.
Use some of your greys and skintones to antialias the eyes and eyebrows., and add some ears while you're at it.
Good luck on this!