Just a question. At what age can kids start wearing thongs? And as parents, would you be willing to buy them thongs? I started wearing thongs when I was 11.
As underwear, I'd just ask at what age girls start wearing thongs and up to what age is accepted they wear thongs. The same answers would be applied for a boy, but as male thongs are not very accepted in men at any age, if I were a father I'd tell him to wear it only at home or with the family, never to the high school, as much with his closest circle of friends if there were enough confidence, because during the teenage if you do something different you're at a high risk to be bullied. As male thongs are not easy to find, if my hipotetic son were not still grown and there weren't thongs for his size, I'd tell him to wait 1 or 2 years until there were for his size.
As swimwear, I'd first ask if is appropiated for a teen girl to wear a thong bikini. If it is, I'd apply the same for underwear.