Nekovalkyrja (pronounced “Nay-ko-wul-ku-ree”) are the cat-eared clone soldiers of the Yamatai Star Empire.
The idea of a generation is human concept that doesn't apply neatly to Nekovalkyrja because they can produce at a young age, and (as you noted) they are a very young race in general, with Nekovalkyrja not even being heavily mass-produced until somewhere around YE 20, meaning most Nekovalkyrja are younger than 15. If I had to break down Nekovalkyrja into generations, it would be by model numbers or by the wars they were produced for.
The NH-33 Series Nekovalkyrja model is the most common, followed by the NH-29 Series Nekovalkyrja and the NH-27 Nekovalkyrja. The NH-28 Nodal Integrated Weapon System is uncommon but still in limited service. All other nekos are either extremely rare or obsolete and replaced.
Originally, the Nekovalkyrja series was developed by PNUgen Corporation for the Uesureyan Empire. Later, Nekovalkyrja production was assumed by the Ketsurui Zaibatsu.