It doesn't matter to me at all...I once read a question about a girl who hated wearing normal panties when she was little so her mom bought her thongs. I think it doesn't matter what you wear down there, as long as a little girl would be wearing it for comfort and not sexual reasons.
As for when to start it doesn't matter. What is important is why. If at age 11 she wants to do it to attract boys then no. If she wants to do it because she thinks they would be comfortable or she wears tight clothing like leggings or spandex pants and doesn't want panty lines or she wants to act like mommy. Then sure. But she has to learn the thong does and donts. Really it is up to the parents if she is responsible enough and is wearing them for the right reason. As far as old. You are never too old. If you stay fit and dress nice then why not. No one sees it but you and your significant other.
I think the age that is too young for thong underwear would be like (I'm just giving a range) 8-11 or 12 and the age I think is too old to wear thong underwear would be (Again just giving a range) Late 30's/Early40's unless the person happens to look young and good looking. But despite my opinions there is no true law or rule stating too old or too young for thong underwear it's up to the person/parents to decide whether they want to wear it or not and we have to respect that.
i can't remember when I first wore one-might have been around 14 and I only did it because all the other girls in school did and I think 14 is an ok age to start. I think too old is 40+. Personally, I stopped wearing them at 16 when I left school because they're uncomfortable and I don't think they're sexy at all-i hate them now I think boy shorts are much cuter.