If you're a teen, hopefully you've lived in a way that instills a sense of trust of you in your parents. If so, then it should be no problem for you go to them, ask them.
So, you're asking what is the ideal age to let your daughter fit herself in that thong underwear? Many of the teenage girls are pleading with their parents to have.
Parents routinely need to address clothing styles and what they consider acceptable for their teen to wear. This is a page about "When should teens wear thongs.