Why not? It’s UNDERWEAR, not for public display and very much your own personal choice.
What you wear as underwear your own personal choice, but for some reason others think society at large should control what you do and wear, even under your cloths.
If you are religious you may have to deal with the fact that some religious people believe that as personal servants of god, they should always wear the underwear that their profit prescribed. Mormons are big on this, they have magical underwear that the faithful wear religiously. If you are a member of one of the religions where the minister or church leader has proclaimed what underwear his god wants you to wear, then you do face a moral choice, other wise the choice is just your own.
Then again there are parents. Every time you make a rational choice they are likely to step in and give you the ‘I know what’s best lecture’, which would be fine except you are no longer five years old.
The question is not if you are old enough to wear thongs, which are after all underwear and not visible to others, but are you old enough to wear hats? You may have noticed that some adults wear hats but children do not, except when they are dressing up and pretending to be adults. If you think you are old enough to make the choice of wearing a hat or not, then you are old enough to select your own underwear.
Oh most definitely! 15 is well over the age of an average girl’s first thong. I got my first one when I was 13. You would save your child the big scare and embarrassment of having her classmates see her underwear line under those tight jeans too! she will thank you!