Montgomery GI Bill for the 21st Century Act - Reduces, for purposes of certain student financial aid computations, the expected family contribution for individuals receiving educational assistance under the Montgomery GI Bill (MGIB) by an amount equal to the statutory reduction in basic pay or voluntary separation incentives for such individuals.

Authorizes enrollment in the MGIB basic educational assistance program by certain individuals who participated in or were eligible to participate in the post-Vietnam era veteran's educational assistance program.

Provides for commencement of the ten-year delimiting period for the receipt of certain training program benefits by veterans, survivors, and dependents upon commencement of such training.

Makes MGIB educational benefits available for payment of fees associated with national admissions exams and national exams for credit at institutions of higher education.

Increases the maximum loan guarantee amount for veterans seeking assistance under the Veterans Administration home loan guaranty program by indexing that amount to the Freddie Mac conforming loan limit, adjusted annually.