If I am under 18, are my parents responsible for my student loan?

No. You are responsible for repayment of your student loans. You parents are not required to cosign your federal student loans, even if you are under age 18. The 'defense of infancy' does not apply to federal student loans. (The defense of infancy presumes that a minor is not able to enter into contracts, and considers any such contract to be void. There is an explicit exemption to this principle in the Higher Education Act with regard to Federal Student Aid.)