Immigrant Question (WWII Era)

Factual Questions
1

This is from the same World War II era movie I saw (I posted another question). Anyway, at the movie (a comedy) there is the usual WWII propaganda like how we should all buy war stamps etc, but the male lead says

“And don’t forget the air force is looking for young men who want to learn to fly airplanes. This 12 month course is open to any male in good health, who has a high school dipolmal, and is between the ages of 17-27 and has been a citizen of the United States for at least 10 years.”

The last part is what got me. I don’t know what the requirements are but do kids as young as SEVEN or as old as SEVENTEEN usually become citizens. It seems like that last part about being a US citizen for 10 years must be serving some purpose to eliminate a lot of men but I can’t figure out what. Or am I reading too much into this?

So is the part about being a citizen for at least 10 years have some meaningful purpose during WWII was it an attempt to eliminate people?

2

Children under the age of 16 automatically became citizens of the United States when one or both of their parents became U.S. citizens. Well, not quite automatically: the parent had to list the children on his or her citizenship application.

3

That sounds like a requirement for flight training. An awful lot of resident aliens were drafted.