This wasn't so much an offensive question, but an interesting assumption nonetheless.
BG: I've mentioned before that one of my dearest friends is in her 50's, and I'm 31. We met online and hit it off because of a shared love for Pirates of the Caribbean, Johnny Depp, and a love of all things Irish. Plus we have the same taste in clothes and jewelry. Most of the time when we're talking online, on the phone or in person we don't notice the age difference. She doesn't look her age, either, as she looks more like she's in her early 40's, at most.
I went to meet and visit her last summer in her hometown in Iowa last summer and we had a grand time shopping and just hanging out.
Here's where the interesting assumption comes in. A few places we went to outside of her hometown (It's small and they all know her, her sister and niece) to go shopping, she'd tell them I was visiting from Maryland and joke about our age difference and they'd ask, "Are you two related?" We'd tell them "Oh no, just good friends."
We don't look a lot alike apart from having fair complexions and auburn hair. But then again if you saw her and her niece together you wouldn't necessarily guess they were related if you didn't already know. And I don't look like all of my relatives either.
It's just an interesting assumption, I think, that two people hanging out who are a significant age difference are probably related.