There are a few markers of adulthood that are unexpected, but I believe once identified are unmistakable.
The first is the often quoted “You know you’re an adult when you have a plastic bag full of plastic bags”.
but I’m going to add my own thought to the internet-verse.
You know you’re an adult when you realize good quality blankets are really expensive.
In my early 20s, I got rid of a whole bunch of blankets that had been handed down to me because they weren’t my style. I probably should have kept about half of them. My linen closet looked great though.
I’ve been trying to replace my kids lumpy polyester comforters for a little while, when I saw this beautifully made Pottery Barn queen-sized quilt for $8 (yes, you read that right. Eight dollars):

I texted eldest daughter to see if she liked it (although truthfully, I probably would have bought it anyway). She did indeed like it. Sold!
I had some work to do on it once I got home. There were some stains to treat, and it had clearly been dragged on the floor in the back room. I ran it through the washing machine twice for good measure. And it took forever to dry.
But my results were fantastic.

Pottery Barn Kids queen quilts usually range between $100-200, so I think I got a great deal. And now my tween has a quilt with no Disney princesses on it.