I really don't remember how old I was when I got my first thong, except that I was in college, so in the mid 1990s. My first one was a Jockey thong I bought at JCPenney. At the time, they were sold in tiny boxes that contained one pair. I think it was something like $10.00, which seemed like a real splurge for one pair of underwear. To be honest, I didn't even really know what a thong was. I liked how it looked on the model on the box, and decided to try it. The picture only showed the front of course, and when I opened it and saw the back, or lack thereof, I wondered what I had gotten myself into. I tried them on for the first time, and honestly, I wasn't sure I really liked them. But I tried them a second time, and then a third. I was hooked.
Thong swimsuits came a bit later. I got my first one in 2000, while on vacation in Washington DC for the Millennium March on Washington. It was one of the now long gone Speedo thongs. It took me three more years to actually wear it on the beach, during another trip to Florida. Again, I was hooked. I had one swim thong at the beginning of the trip, I had three by the time it was over. Now, I couldn't even tell you how many thongs I have, other than to say "A lot."