Although you are still a youngster, by the appearance of these sketches it is obvious that you have had appreciation for your skill for some time. To critique what is shown the only improvement that I first see is the muscles you have drawn in your hands. I will leave the rest to your own observation skills since, you have been doing well with them.
It is difficult at times for vocational artists to understand why they create what they do. This is good reason to continue drawing. Try to use the best materials you can afford and start storing your drawings where they can be protected from the elements (in books, flat files, etc.). As you progress into different mediums or subjects building a portfolio refer back to them occasionally and you will begin to understand the effect these sketches had on your abilities for future work.
Recommended read: Dynamic Anatomy: Revised and Expanded Edition By Burne Hogarth