> Countries that are only a century old aren't legitimate political entities?
The concept of nation usually revolves around having a common cultural background. In less than a century, I'd argue there may still be significant differences in the fabric of the country, and what may have been true 100 years ago when they creating the State may not be true anymore (i.e. people may want to break free from it again).
> The concept of nation usually revolves around having a common cultural background.
Sure but the idea that "nations" ought to be coterminous with sovereign governments ("states") is a fairly novel one that's never been all that well followed (except that since it became popular, states have tried really hard to create the idea of a coterminous nation as a means of inspiring loyalty.)
How new a state is has no relation to whether the state corresponds to a nation; there's plenty of nations that have never had a state, and states that have been around a long time that don't correspond particularly well with a single nation.
The concept of nation usually revolves around having a common cultural background. In less than a century, I'd argue there may still be significant differences in the fabric of the country, and what may have been true 100 years ago when they creating the State may not be true anymore (i.e. people may want to break free from it again).