Frank is correct, females under age 21 are ineligible to be members, since they can't meet the membership requirements.
Wagionvigil is wrong however. ASM's (or Asst Team Coaches) under 21 can be elected as youth members, if otherwise eligible.
Here are the specific rules from the Guide for Officers and Advisers 2008 (24-413):
Registered active member. In Boy Scout troops and Varsity Scout teams, every registered active member of the unit under age 21 at the time of the election is eligible to vote.
Youth membership qualifications. All members of, or candidates for membership in, the Order of the Arrow who are under 21 years of age shall be considered youth members or candidates for youth membership, subject to meeting the following requirements:
* Be a registered member of the Boy Scouts of America
* Hold the First Class rank of the Boy Scouts of America as a minimum
* After registration with a troop or team, have experienced 15 days and nights of Boy Scout camping during the two year period prior to the election. The 15 days and nights must include one, but not more than one, long-term camp consisting of six consecutive days and five consecutive nights of resident camping, approved and under the auspices of the the Boy Scouts of America. The balance of the camping must be overnight, weekend, or other short-term camps.
Candidates for youth membership shall be elected by other youth members in accordance with policies set forth by the national Order of the Arrow committee.
Adult membership requirements. All members of, or candidates for membership in, the Order of the Arrow who are 21 years of age or older and who are registered members of the Boy Scouts of America shall be considered adult members or candidates for adult membership. ...
Paul S. Wolf, P.E.(Ret.) mailto:firstname.lastname@example.org
Secretary, US Scouting Service Project, Inc.