Even if your premise was true, which it isn't, do we justify the imperialist invasion of a sovereign state because it was founded after some threshold year?

Countries are not a fact of nature. They change, come and go for a large number of reasons. The Ukrainian people are constituted in a country and have repeatedly expressed they don't wish to be part of Russia. If you want to force them to be at gunpoint, at least be open about it instead of hiding behind a historical pseudo-argument.