Evan -
I am an Army MARS member, but I think that I can answer some of your questions.
Multiple callsigns - Every MARS member receives a distinctive call sign. Individual Air Force MARS members start with AFA, Army with AA, Navy with NNN. The numbers what follow indicate the Region. Club and military stations also receive calls. The second callsign indicates that the individual occupies a billet (AKA, position) within the organization. For example, I have the following calls assigned to me:
AAT3BF - individual callsign
AAM3O - Emergency Operations Officer, Eastern Area
AAA3DE - Delaware Army MARS Director
AAA9AC - Army MARS Automation Coordinator
Each service has a different scheme for the assignment of billet calls.
The age thing. I would assume that AFMARS is the same as Army - we will accept 17 year old members with a parent or guardian's signature. Without it, you must be 18 to join. We do, however, accept Auxiliary members who are younger.
73 and best wishes in your MARS experience,
john
W3JKS/AAT3BF/AAM3O/AAA3DE/AAA9AC