In my grandparents' house of 35 years there is a little room in the basement which has served as kids playroom/ extra bedroom over the years. When I was in 5th grade, my cousin in 6th, and my sister in 7th, we got permission from Nanny to paint a mural on one of the walls. We painted a jungle mural, complete with giant black panther, a parrot, a snake, and lots of trees and flowers. Its still there
It covers one entire wall. We were actually pretty good! During later years some of the younger cousins took to writing graffit style all over the other walls of that room and while it doesn't look anywhere near as good as our mural, it's still interesting to look at now.
I currently rent, so it isn't an issue with my kids. No way would I let my 3yo scribble randomly all over his room, but older kids, especially if they were serious and had a good plan of what they wanted to do, quite possibly!