A couple things stand out (and for context I turn 23 in a few weeks).
When I turned 20, a friend said "congratulations, you're half way to 40!". I have since returned the favor to everyone I have come across on their 20th birthday.
Finding out a girl I *thought* was in middleschool, was actually a senior in highschool.
Finding out a girl I thought was in college, was actually 14, and feeling creeped out about my mistake.
(I have since stopped guessing at the ages of females)
"Next up on our show, we'll be playing the best of the 80's and 90's"
wtf, we aren't in the 90's? 🙁
But the one that made me realize my childhood was gone, was watching a little boy, probably 8 years old, in full run jump off a 4 foot wall and land in the grass, falling to his knees, and without pause gets up and keeps running. I've played sports all my life, and still play Soccer and Ultimate frisbee. I even mountain bike and snowboard on occasion.
But this kid obviously thought nothing of jumping off a wall taller than him and landing on his knees. I remember watching him and thinking "ouch, why didn't you run around it!" and yet I remember jumping off things for the fun of it. I knew a part of my childhood had been lost, right then and there 🙁