Hi! I'm only a teenager myself, but I'll try to do my best.
First of all, I think that draeing of your friend turned out very nice! The only thing I want to mention to that, is that your drawing style is quite loose (just like me), but when you do the shadow, some of the lines can be mistaken for wrinkles, and that makea her face look kind of old, even though all her othe features point towars youth. Also you could have added a little more contrast to her hair so that it gets a little more volume.
I also think that you are far on your way to deloping skill with the two body drawings! They are really great, and you really show abd define where the muscels are, but on both drawing I get the feeling that the legs are too short, compared to the torso. Also I feel like the hand look a little lifeless and stiff, which is something I also struggle with alot. Maybe you could practice drawing some quick hands, just to learn how they are supposed to look. You can also look up a tutorial or something about how to add life and motion to your drawings.
I'm not sure if this was helpful, and I actually think you draw a lot better than me, but at least I tried :)
1