Maybe your parents feel guilty and if you get stoned and get hit by a car they could never live with themselves... They want you to at least be 18 so they can justify it if something happens... Or maybe they feel they fucked their lives and don't want you to fuck up yours...

Maybe they know the tiring mind numbing effects will interfere with your school work and studies,let's face it, a 13 year old stoner is prob not going to be a brain surgeon...

Look at the course you are taking an plan what you want to be and then figure if weed will help??? If you can't do that then listen to your parents till you can...