In my personal headcanon, I see her as trying out for the Wonderbolts as early as the age of fifteen, while she and Soarin are freshman in high school and they are so impressed by her skills that they ask her to join them despite her age as she tries to juggle her career with schoolwork.

And not too long after gets promoted to Captain, and the reason she doesn't allow Dash to join just yet, is because she sometimes feels as though she was allowed to join and become Captain too early.

1812276
I see these things a bit too differently. I view the Wonderbolts as a aerial military unit that... I can't think of things that they would do, but still. I think of her trying out at the age of 18 and being placed on the kind of, not really reserves of the group, but not the main show of the group. She then gets better over a couple years and becomes one of the featured members and, after a few more years still, ascends to the rank of captain (about 25 years old).
And then, for why Dash isn't allowed to join yet, I don't really have anything quite yet for that. I could possibly argue that she isn't mature enough or that Spitfire is a little afraid of losing her position to such a new Wonderbolt. There are countless possibilities, and I'd love to hear even more of them.

~SolidFire

How did Spitfire join the Wonderbolts? Well, that's a no-brainer. Hard work and dedication. Doy.

I guess if you wanted me to go into detail, I'd say that her wingpony at the academy was Soarin', and she proved herself to be an ideal leader. Eventually, she rose through the ranks and became captain. It's really not that complicated.

Why isn't Rainbow Dash a Wonderbolt yet? Also a no-brainer. Everypony has to start somewhere, and RD is starting as a cadet. Being made a lead pony was a big step, and she has a ton of potential. But she still has a lot of growing up to do in order to be accepted into the Wonderbolts' ranks.

1812704

Once again, my comrade, your theories make brilliant sense.

my personal head canon is that she decides to attend their school, not the camp like Dash, the actual academy, but just by herself. Soarin stayed in regular school, I don't see Soarin as one to go through a military academy and if he did then they were not partners, they were mixed around so they could learn how to work with others effectively. she graduated but that didn't get her an automatic spot in the Wonderbolts, recently graduated ponies have to wait a year before they can tryout. so she practices with her friends and then they try out and get in. She works her way up to the Captain position through dedication and hard work, in the present day I see her as about 10 years older than Dash, so she's around 26-30 years old.
My reason for why she doesn't let Dash in: Dash isn't old enough and frankly still not mature enough, yeah she improved but Dash's initial reaction to being made a wingpony didn't sit well with Spitfire. and she doesn't think Dash's acrobatic skills are good enough yet, she see's that Dash is fast but thinks that she needs to work on her acrobatics.