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.