My wife: Growing up, her mom just didn't allow thongs, so even though she was aware of them in high school, she only wore regular "boring" underwear. Went to a big party school for college where it was obviously just the standard for all girls, so she made the switch then. From that point on (18 years old), that was exclusively what she wore. Switched from fullback underwear to thongs because of VPLs in yoga pants.
College girlfriend "M": Classic 2010-era preppy girl, basically lived in a combination of Hollister and Abercromie clothing. Loved to wear cute thongs (exclusively VS/Pink). Started wearing thongs partway through high school and never again wore anything else.
Post-college girlfriend "B": Worked at Victoria's Secret during high school, and to say she was obsessed with the brand would be an understatement. Started wearing thongs freshman year of high school. Was so obsessed with thongs that she was always trying to convince girls that weren't wearing them that they needed to convert.