Heard about it in passing over the years, but didn't know what it was. Eventually run across the dichotomies, probably online, but can't wrap my brain around it, so then keep moving. I get into temperament (The APS system I talk about) through my wife, at 38. Two years later, someone introduces us to Keirsey, and from there, I learn about the MBTI system. So this is 40. It took almost another year to settle on a type. 46 now.
I'm so into it now, but seem to be such a late bloomer compared to others. I wish I knew about it when in teens, as it explained a lot of things, including part of my "weirdness" (heavy S environment). All I knew back then, was from the discussion my father gave one time about introversion and extroversion. And then, I noticed among the extroverts, that some were more light and airy, and others were more serious. For introverts, it was harder to tell the difference. He also discussed "head" vs "heart" another time. And he favored "concrete" reality over too much "abstraction".
IT seems all of this stuff is so implicit in life, even though many do not take it seriously.