There is a lot that factors in to a coach choosing a captain.
-Role model behavior : Are they on-time to practice? Are they responsible? Are they always encouraging their teammates and leading by example? Are they a leader at practice as well as outside of practice, i.e. grades,etc?
-Leadership abilities: Can they inspire their teammates? Do their teammates respect them? (otherwise girls will not listen to the captain) Can they handle high pressure situations?
-Communication skills: Will they keep their teammates informed about practices/reminders/what to wear and bring for games and pep rallies, etc.? Will they be able to communicate ideas/thoughts/feelings between the squad and the coach? Will they be able to speak up when needed? How will they deal with a teammate that is having a bad attitude?
-Experience: Have they shown that they earned the spot through hard work in the past? Have they done the extra tumbling lessons/other allstar or school cheer teams in the past? (you don't have to be the most skilled, but teammates have a hard time respecting someone that is the weakest on their team typically. Therefore it helps to have above average skills or at least a solid, dependable average of skills). Age will play a factor as well.... If two candidates are exactly even on their potential, the older one will probably get chosen (Senior over a Junior or Sophomore for Varsity Captain- Junior or Sophomore over a Freshmen for JV). If they are not even though on their skills/behavior/abilities/personality, then a younger candidate could win out.
Just keep working hard and showing that you have the leadership potential, and eventually you could earn yourself a captain position, whether it be as an 8th grader or a 12th grader.