For me it was two summers ago. I was only 19 at the time, but for the first time ever I realized that I'm never going to get any younger, and I'm not going to stay youthful for the rest of my life. Obviously I technically always knew this fact, but it never hit so realistically until then.
I was working as a tutor for a summer program, and I said something about "Peony Park", an amusement park that was torn down in the early 90s here in Omaha where I live. Peony Park was a local treasure, and there are several generations of Nebraskans whose childhoods were spent there. Anyway, one of the students I was tutoring said, "Peony Park? What's that?", and the realization that these kids hadn't even been born until after the park was torn down hit me hard. For my friends, my parents, their parents, and beyond, Peony Park was just...Peony Park. Everyone knew what it was, and those who didn't...well...there wasn't anyone who didn't! It was such an odd experience for me, and it's stood out as a turning point somehow.