I guess there's really no age at which you become too old to wear a thong, just an age at which nobody wants to see you in them anymore. Well into her 50's, Madonna is still flashing her booty in a thong, in this case, on the pages of Harper's Bazaar magazine.
As to whether it's time for The Material Girl to hang up the thong and maybe put on a housecoat, I'll leave that decision up to you. She's obviously still hitting the gym pretty rigorously, and while you can't ever completely fight Father Time, I can see an argument going either way for granny to keep or ditch the thong. Enjoy.