We've had more than one of our kids, male and female, as young as 15 years old, arrive at BKK unaccompanied on more than one occasion over the years. It's never been a problem, on any level, whatsoever.
That said, in the interest of doing everything we can to avoid any potential complications, whenever our under-18s travel alone internationally, we always make sure they carry a written travel authorization, signed by both parents, and containing every possible contact point (including grandparents' names and numbers in case we can't be reached).
Regarding money, none of us has EVER been asked to demonstrate ANY cash in our possession upon arriving in any country, much less Thailand.
Your woman-child should be fine.
~mrs