Usually if someone is a bender they'll start bending as a kid. That's about the only way honestly. Some families just have more benders - like the royal family for example - but it doesn't mean two non-benders couldn't have a bender as a child. At least in the Fire Nation, I don't really know about the others.

Not really, defense was never a big issue. I was a politicians daughter, I was pretty much protected twenty-four hours a day whether I liked it or not.