I personally believe that weight gain as we age is normal. This doesn't mean that everyone will become overweight as an adult, but as you mature, so does your body. Our bodies continue to change throughout life...every part ages....it just is.
It's great that you are concious of your health and staying active now that you no longer do ballet. You're setting up good habits for yourself to follow as you age. However, you certainly don't need to worry about your weight at 5' 4" and 107 lbs., but to be aware of your health and maintain your level of fitness at any age is wise.
All the best!
If you are persistent, you will reach your goal. If you are consistent, you will keep it.