FWIW, I was the horrible neighbor that told the other kids next door at age 5 that it really was your parents, not Santa. Uh oh.
I remember not feeling surprised when I figured it out, just more like I knew it was 'pretend' all along in the name of fun. Oopsie to ruining the neighbor's fun! urk.
I had two older brothers, one by four years, so I honestly don't remember a time when I believed in Santa. But I am the kid who sat down in Santa's lap at the kids' Christmas party at the local American Legion, and let out a loud and surprised, "HI, Uncle Kent!"