Well, just by doing some quick math, it would seem America ages 1 year every 12.1 years. The US was 230 years old in 2006, and if you divide 230 by 19, you get 12.1 (with rounding of course).
As for England, that's a lot harder to determine. After searching around, I couldn't find anything definitive telling me when England actually became a country. The United Kingdom was formed in 1707, which would make it 299 years old in 2006. 299 divided by 23 equals exactly 13. If you went by that, then you could say that England ages 1 year for every 13 years.
The name "England" came from the Anglo-Saxons, who took control of England between 500 and 1000 AD, but there's no exact date. There's also the period in 1066, specifically December 25, when William the Conqueror was acclaimed king of England in Westminster Abbey. This could be considered the first time in which England was actually viewed as a nation.
Using the 1066 date as the beginning of the nation, you'd divide 939 by 23, which gives you 40.8 (again, rounded), so the character would age 1 year for every 40.8 years. Note: I used 939 instead of 940 because the country didn't turn 940 until December 25 of that year. The difference is negligible anyway. It would be 40.9 instead of 40.8 when rounding.