User blog comment:Yuikikawaii/their age in hetalian years/@comment-1663160-20101204033844

There is no formula for their ages. Nations age at a somewhat random rate. They grow when things start happening to them or simply grow over time. As shown in canon, America grew to young adulthood in what, to England, was a very short time. As a result, their ages simply are what they are based on factors of both history and what will be their futures. Their bodies know when to age because of happenings. America grew up quickly because of America's quick development in reality(and likely because of the Revolution.)

And another thing, rudeness unintended, but don't you think that, if there was a formula, it would work for all of the nations, not just a few?

Their human ages will likely be given over time, however we already have several, as you can see on their profile pages. As appreciated as your effort is, Hetalia is to be taken mostly with a grain of salt(even Himaruya's admitted to there being inaccuracies and things he did simply because it was "cute"; it's called artistic/historic licence), and thus using formulas is a little. . .foolish?

TL;DR: Like everyone else said, no, that's not right at all, some of their profile pages have their actual ages. There's no forula for this and, if there was, it'd work for everyone, not just a few nations.