While you don't have to use this guide to decide how old your character should be, if you have no earthly idea and can't decide then perhaps this will help. These are just my headcanons about how it works (and it can also apply to states and other national subdivisions). So if you'd like to borrow my headcanons, feel free!
General Principles - Nations always grow older, never younger. If their situation changes to a stage earlier than they appear to be, they simply maintain the appearance of the latest stage they've ever been in. A nation's age is not very much connected with time at all - they may stop aging entirely, age slowly, or experience a sudden growth spurt depending on conditions.
Chibi Age/Very Small Children - These are very young nations/states/colonies/etc. They certainly aren't recognized as proper nations by the world, and in fact are usually seen as belonging to another nation. Or they could be fending for themselves as best they can, and in that case are easily claimed by another nation. They don't have a proper government - if anything, they have some sort of informal internal management.
Older Children/Pre-teens - At this stage the nation/state/colony/etc. has an organized governing system, but they're still very much in need of help from older nations - and they're either okay with that or they're not in any position to do anything about it.
Early Teens - At this stage the nation/state/colony/etc. is not okay with being so dependant. If they officially belong to another nation then they begin making demands that demonstrate this feeling - they want better control over their own internal affairs, they want representation within their older sibling's government, they consider themselves to be their own nation already even though nobody recognizes them as such, etc.
Mid Teens - This is the first stage at which a nation may be recognized internationally, though they're not necessarily so. They're pretty independent, but obviously not fully so and thus aren't quite treated fully seriously by older nations on the world stage. They may be relying too much upon an older nation economically, or for defense, etc. or sometimes they have to team up with other nations in this stage to manage such things. Sometimes they're just too caught up in internal affairs to have the time and resources to do much about the outside world.
18 to 19 - These are nations who are strong enough in their own right to be treated fully seriously on the world, but are still considered to be quite young in the grand scheme of things. Older nations look down upon them as not having the same rich and ancient culture spanning centuries as they have.
Early to Mid Twenties - These are nations who have been around long enough to have a truly deep national culture that is distinct from any other nation.
Mid to Late Twenties - These are nations whose cultures don't go back centuries but at least a millennium. They tend to be quite respected by other nations for this - it isn't every nation who can manage to last for so long.
Thirties - It's always a very bad sign when a nation reaches their thirties - it means that they're actually showing signs of aging. Their culture may have become stagnant or their people are falling away under the influence of younger nations, and some sort of a collapse is inevitable, though it hasn't happened yet. In fact, it may take a very long time for this to happen - centuries, even - but the nation has become locked into a situation from which recovery is impossible.
Forty Plus - These nations are no longer really nations, having lived out their span. They're retired, but the fact that they've not disappeared entirely indicates that their culture has not yet fully disappeared. In fact, it's not unheard of for a nation of this age to raise up another nation, though that's rare.
