I realized I was a grown-up when we brought my daughter home from the hospital. I walked upstairs and found my wife clipping the baby’s fingernails.
I remember thinking, “holy shit, that little thing can’t even clip her own fingernails” and realizing just how responsible we would have to be.
I had a similar realization when having some beers with old friends, having a blast, and realized we were talking about recent appliance purchases.