What was it? For me it wasn't graduating high school, joining the military, having a kid, or anything like that.... Mine was buying furniture. Up until my dd was 2 I had hand me down furniture and dishes. After an injury settlement from a car accident I had some money. I donated all my old furniture to a family who lost everything in a fire and bought new furniture that I picked out, negotiated for, and paid cash. I finally felt like an adult.
What was yours?