it is normal to get weak veins in that age. i knew 3 girls with that problem when i went to school, at the age of about 18 and just one of them had "luck", she had only spider veins. the only thing that really helps is: keep moving!

You need to get a routine like having morning walks (no running!, just fast walking up and down hill, if possible), swimming at least once a week (you'll see that is the most effectiv help; do rather crawl than breaststroke) and whenever you like or feel pain do some stretching like yoga.

last: be carefull with muscle training. Why? Just look for damaged veins at the arms of bodybuilders. when the veins are weak such sport might be too much as well as jumping including running, tennis etc.

I hope i could help you and forgive me my grammar