What is an appropriate age to give your child boundaries and Independence when in a public place? : Parenting

My wife and I took our six year old and eight year old to see a movie tonight. After the film, I took my eight year old to wash his hands while I drained the vein. He finished before me and so, having complete trust in his ability to find his way back to my wife and his brother, who were around the corner and maybe 20 yards down the hall, I didn't give it a second thought when he left the bathroom. I walked down to where my wife and son were when we'd left for the bathroom, which was on a couch right outside the theater, and no one was there. I called my wife. No answer. I went back to the bathroom. Not there. Went out to the car. No one there. Wife calls with both boys and she's upset that I left my son by himself. I asked her why she left where she was and she said her and my other son went back into the theater and when she came out, my eight year old was by himself on the couch where we all were before we left for the bathroom. She got angry at both me and my son, and when I told her he did the right thing and she should've stayed put where she was, considering that even if he and I were together, we wouldn't have known she went back on the theater in the first place, she fucking blew a gasket saying it doesn't matter where she went, I should've never left him alone and that he could've been kidnapped. It took everything for me to refrain from telling her to stop coddling him, but I didn't, thinking that maybe I'm being irresponsible by letting him go 30 yards away from me to his mother by himself. When did you, as a parent, start allowing your children freedom and independence to stray from you, in public places, knowing they knew how to get back to where they were supposed to go?