Excellent, I look forward to seeing this character. ^^ 

And...

To Quote my edit you may have missed:

[b]"[/b] I'm quite in the muse for writing the mercenaries right now, however, I'm trying not to get ahead of myself and write more than works. My plans were to make the mercenary troop itself be quite old and the Captain would only have inherited it (so to speak). He's going to be ~28 years old (for note, six years older than Ismaire). I was planning on having him inherit the rank at a fairly young age due to being the most nominated/strongest/qualified/whatever after a rather unpleasant defeat during which over 50% of the troop was killed. He was going to be anywhere from ~21-24 when he becomes Captain. Going with this I see two (or two and a half) major points for her to join.

[b]1.[/b] She joins before he takes the role of Captain. This could allow her to join at the ages 15-18 (maximum) and I could just adjust his age of succession to fit however long you want her to be in the group. (Say you want them to have both been in the group four years before he became Captain—she could join at 14 and he could take the position at 24, etc.). 

[b]2.[/b] She joins after he has already become Captain allowing her to join at ages 15-18 (minimum). She could join either almost directly after his promotion to Captain having her still be with the group a good while [i]or[/i] sometime much closer to the start of our story, making her more of the ‘new’ member.

Personally I like the idea of her having been with the troop for at least a little while now. Of course if you don’t like the sound of any of this we can do something entirely different. [b]"[/b]

This information is vital to how soon I finish my next character.