Keep in mind that any military organization is setup with a pyramid leadership structure...more junior folks on the bottom, fewer senior folks as you move up.

In theory military rank is granted not just as an attaboy or pay raise, but to enable you to move up to the next layer in the leadership structure. On active duty once you get promoted you will not stay in the same job/unit for long, you'll be sent off to bigger and "better" things.

In guard/reserve units this gets a little diluted and you do see more senior (older) folks staying in one unit forever, even if there's technically not a justification for someone of their rank in that position. This is due to the nature of guard duty and geography.

Most or all AF guard and reserve units do hire a few folks off the street, this keeps a little young blood at the entry-level since they start as LT's and also maintains ties to the local community (as opposed to having a bunch of airline pilots who just commute from other states).