I'm not sure if "purists" would consider it a convention, but I did go to Star Wars Weekends at MGM Studios a couple of times. (I absolutely refuse
to call it Hollywood Studios!) People came in costume, they had cast and crew members doing meet & greets and autograph sessions, interviews, special events, etc. So yeah, I would consider it one.
Also, when I was about seven or eight, I went with an uncle of mine to a ham radio convention. I don't remember much about it, though.