Once in my early 20's, I went to the Officers' Club for lunch with my Dad, and several people thought I was his new wife! Dad's 30 years older than I am :-).
LOL!
Well.. my step-daughter and I were 6 years apart in age difference.
So people automatically assumed that we were 'an item' when we were together as a group.
(She, her mom, and herself.)
And then.. THEN step-daughter would get this evil gleam in her eyes and say to me, "DAD-ddeeee!" or "FA-ther! Just WHAT are you..[insert horrible inside joke here]"
And people's eyes would bug out.
It had to be the biggest and best 'practical joke' ever pulled, and no-one had to set it up either.