if you are 30 but less than 40, you may apply if you are prior service enlisting in paygrade E-4 or above.
What are the qualifications to join the Reserve?
Generally: You must be between 17 and 40 years old. You must not be employed in a civilian occupation essential to US interests during time of war. You must not have served more than 13 years of active military duty. *Note, there are other requirements and restrictions. Qualification for certain Reserve programs may seem complex. For example, if you are less than 28, you may apply for all Reserve programs except RX; if you are 26 but less than 36, you may only apply for the RX program; and if you are 30 but less than 40, you may apply if you are prior service enlisting in paygrade E-4 or above. Please talk to your Recruiter for specific information applicable to you.
RX program - This program is designed to recruit and train non-prior service applicants who are at least 26 years of age, but under 35. They must possess skills that will qualify them for enlistment as E-4 or E-5. RX Members must complete two weeks of Reserve Enlisted basic training . This has to be completed before reporting to a unit.
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Hope that answers the question