Normally, a minor cannot be held liable for a contract that they sign. However, in 1992 the Higher Education Act was amended to permit eligible students, defined as per Title IV regulations, to sign promissory notes for their own Federal student loans. As such, student loans represent one of the few exceptions to the so-called "defense of infancy". The specific citation is section 484A(b)(2) of the Higher Education Act of 1965 (20 USC 1091a(b)(2)), and applies to Stafford, PLUS and Consolidation Loans. It does not appear to apply to Perkins and Direct Loans, although it was clearly the intent of Congress that it should.
Several states have also passed similar laws that consider minors to be competent to enter into a contract for an education loan. This extends similar protection to private and non-federal loans. All private education loans require a cosigner when the student is under the age of majority, just to be safe.