Keep in mind that the rules I’ve come up with here are not
canon. They’re simply just me compiling examples that form a consistent logic.

Overall, I’ve noticed two principle factors that causes the
nations to mature. There are other sub-principles that fall in tandem with
these categories.

1) They need a strong and autonomous economy, which
usually entails having a large population – If the nation is being taxed as a
colony, then they’re not economically autonomous.

2) They need to achieve political autonomy and
sovereignty (independence) – This includes a self-functioning and officially recognized
government.

Let’s go over a few examples.

America: 

At a young age, due to an early economic boom, we know that
America gained super strength. However, because his economy and legislatures
were authoritatively controlled by England, he grew weak for some time.

Now, notice how under England’s overbearing rule America’s
economy is strained. Not only that, but he looks considerably younger.

Then, look at him after he begins fighting for his
independence; there’s a notable increase in age between these two panels.

We also see this same growth spurt in volume 3, where
America ages exponentially right before he revolutionizes.

With control over his politics and economy, America was then
able to become an adult.

Admittedly, in the “Cleaning out the Storage” strip, America
admits that he feels like he’s becoming an “old geezer.”

Lithuania responds by telling him that he’s merely “becoming
an adult.”

Put into context, I interpret this as America maturing
mentally, as opposed to physically. From the example above, we know just how
dramatically he aged.

Likewise, what you’ll see in a moment is that America was considered
to be an adult well before the 1930s (around the time when the outsourcing
strips take place).

Canada: 

Canada is another good example of this autonomous economy/politics
requirement.

For one thing, he ages far slower than America, remaining as
a young child while the latter grew up at a faster rate. This is likely owed to
America’s larger economy.

That said, this proves the strong role that the total
strength of an economy overall plays
in the nations’ aging process. America and Canada were both under England’s
rule and were subjected to taxes, but because America’s economy [and
population] was larger, he matured more quickly.

The political autonomy rule is realized once Canada becomes
a Dominion (1867). While it’s not complete independence from Britain, it was
enough for Canada to physically mature from a teen to an adult.

Monarchies vs Constitutional Monarchies and Republics:

This is a more tentative example, but I’ve noticed that the
nations possibly could have aged as a result of a changed structure in their
governments.

Before constitutional monarchies or people’s republics, the
government was strictly run by the church and later monarchs, who ruled by
absolute divine right. Basically, the people had no say in their governments.

I mention this because in this strip here, we know that
Austria, France, and Russia are about 17-18 years old during the Seven Years’
war.

We know now that France is 26 and Austria is just a little
younger than Prussia, who’s 20↑

. We still don’t have a confirmed age for Russia
yet.

Nonetheless, the difference between now and then is that
democratic governments today are supposed to derive their power from their people (corruption
and tricky nuances aside, it’s the structure not the pragmatics that matters
here).

Starting in the late 18^th century, this new era
of politics and the subsequent provision of constitutional rights might have
lent to stronger senses of national identities, thus causing the nations to
adapt and age in response.

We also know from France that the nations exist and therefore get
their power from their people.

Latvia:

With the following rules above applied, this explains why
Latvia remains so fragile, small, and young despite existing for far longer
than some of the other nations who are physically
older than him.

Latvia hasn’t historically experienced consistent political
independence and autonomy, and has been subjected to foreign control for much
of his existence.

His economy is also considerably poor. He lags behind in
industrial developments and remains largely dependent on agriculture.

So, before anyone says that Hima characterizes a nation’s
age for seemingly no reason, try applying these rules. I guarantee you that you’ll
find some sort of economic or political justification.