Conversations with strangers

Almost every time I leave the house with all three children (which is often), a bystander isn't scared off by appearances or behavior (slightly less often), and the bystanders want to be social or ask questions, I encounter some variance of this conversation.

Bystander: Are they all yours? (While looking around and counting)
Me: Yep.
Bystander: Wow.  They sure are full of energy  How old are they?
Me: Small is 2, Medium is 5, and Large is 8.
Bystander: (counting again) How old are you?
Me: 83 . . . I mean 30.
Bystander: (counting) Wow, I thought you were a lot younger.
Me: Thanks?

It makes me wonder a few things.  Do they think I wandered around town, gathering up stray children who look alike?  Do they think I started having kids at 12?  Do I really look that young, or are they just being nice to make up for the fact that they just asked my age?