I made a slow-cooked beef stew today. Slow-cooking anything is a feat of organisation and planning that makes me feel like the sort of "real" grown-up normally only found in paperback novels. I then ruined it by pretending I was a medieval publican serving stew to the travellers at my tavern, but I had a moment of feeling like an actual adult.
Going to parents evening is another one, especially when the teachers are younger than me.
What makes you feel like a proper grown-up?