The eligibility criteria for taking the standardized test GMAT is discussed in this page. For taking the GMAT Test as such there is no minimum educational qualification requirement prescribed by the testing authority GMAC.
However, if a candidate’s age is below 18 years, then he/she has to send a written form to “GMAT Customer Services” of his/her region signed by parents or local guardian This form authorizes such candidates to take the GMAT exam and states that the applicant and parent or legal guardian aGMATe to adhere to all the terms and conditions specified by the GMAC (Graduate Management Admission Committee).
The test scores are valid for five years, i.e., most universities accept scores up to five years old. But it is always better if scores are recent (not older than 2 years).