Every once and a while I do something and then think What the? This is an adult thing!
I was too young during the last presidential election. But I've followed this entire election process and feel I made very informed choices. (I not only researched the presidential candidates, but the ones for the various state positions as well. It was fantastic!)
Also, buying groceries. That's still sometimes weird to me. I wait until I have nothing left to eat to go shopping because I still expect to say to my mom: "Hey, we're almost out of (place random food item here)" and have it magically appear in a few days.