It was very hard to answer without sneaking a peek at other answers. Here's what I thought, though!

1. ?

2. Perhaps the child was born before there was a "United States."

3. Mount Everest was still the highest, though undiscovered.

4. She was born in the southern hemisphere!

5. At the time of WWI, they would not have knowledge of WWII, so there wouldn't be a "I" on the sword.

6. ?

7. Every year!

8. ?

9. 9

10. Infinately- if you go into negative numbers.

11. I'm not so great at rearranging letters- it would take me way too long!

12. No clue!

13. "The yolk of the egg is white"

14. I don't know- maybe they don't have any cameras!

15. One person is a man, the other is a woman. They are the parents of the child.

16. Meat.

17. The room with the lions. They would surely be dead.

18. I don't know...maybe he's undead. :P

19. ?

20. Yesterday, Today, Tomorrow

21. ?