I remember seeing the ending to a movie (it was on tv, so it may have been made for tv) where two adults, a male and female, walk into what looks like smoke, and come out as children. Then they walk off.
I was a little kid when I saw this, so I may have gotten it mixed up. That may be why I've never found out what it is. This was back in the very early 1980's. For some reason that scene always stuck in my mind and it bugs me that I don't know what it is.