Convention generally is that you don't need to play your 2.0 character as a 'stranger' to Eorzea. You could have been a GC soldier at the time if you're not playing a warrior of light (And there are other threads throughout the forums that speak to WoLs and their role in things/RP). You'd just have retreated after the order was given by the city-state leaders when everything was going to hell.Â
Fifteen is a 'bit' young to be a professional soldier, though. I assume Eorzea has standard ages of majority. That said, not like there are medieval birth certificates or something. It could be an interesting story beat. Your character wanting to fight the Garleans bad enough to lie about her age, and the GC being too hard-pressed for bodies to look too deep into it.
Fifteen is a 'bit' young to be a professional soldier, though. I assume Eorzea has standard ages of majority. That said, not like there are medieval birth certificates or something. It could be an interesting story beat. Your character wanting to fight the Garleans bad enough to lie about her age, and the GC being too hard-pressed for bodies to look too deep into it.