No, there aren't ages, it just continues, so countries can "carry" there size forward to carry on becoming bigger. However it is no guarantee, there is the ebb and flow of nations, as well alliances forming between nations, grouping into coalitions. For example, I hear before my time in the game, Indonesia was rampaging through the US. Now however, I read in a in-game newspaper, they have fallen on harder times.
There are rankings of the countries, but there is more than one way to rank them, such as Experience points and number of companies and so on. However I will copy here the current top10 in terms of population:
1: Poland 40326
2: USA 25825
3: Spain 21347
4: Serbia 20162
5: Russia 15929
6: Hungary 15849
7: Croatia 13350
8: Romania 12533
9: Brazil 11052
10: Indonesia 8124
Though with new joining all the time, and people moving around, these do change. As you can see it is not very much reflective of real life populations, though this not necessarily a bad thing, and there are always attempts to get people to join one country or another. Also the world looks rather odd, with regions, having started out in the countries there are in real life swapping hands. Also there is the practice of "corridors", where since countries can only attack regions which border their current regions, sometimes countries allow other countries to "conquer" some of their regions to allow the other country a border with a third country, so the second country can attack the third country. Then when they have take some of the third countries regions they allow the first country to "reconquer" the regions they gave them, leaving the third with "islands" of territory, disconnected from their homeland. This explains the Hungarian region in China, or the Americans being in India, which I mentioned earlier.