Well all Neko are considered having reached psychological adulthood upon completion of their socialization training, and born Neko experience the same rapid evolution of cognizance upon their maturity out of the youngling state. Beyond that, there seems to be very little wikified information regarding how Nekovalkyrja perceive their own age or the differences between them. Still, in roleplay, old Neko like Yui or the other Neko Taisho — as well as "Evil Neko" and veteran samurai characters and the like — seem to gauge their own relative maturity based on their experiences just like any regular human would. Which is to say that the older a Neko gets, the more badass and knowledgeable they seem.
A Neko of two or three years would definitely be considered some kind of adult. Whether that young Neko is on par with an 18 year old or a 30 year old in real world psychology largely depends on how the player wants to present them. Conversely, Neko who have seen more wars and lived for longer probably see themselves as being more senior than Neko who were born in YE42 and only saw combat at Glimmergold; I'd assume that the more wars a Neko has lived through and served in, the "older" they are universally perceived by the broader Nekovalkyrja society.
I don't think there's any hard-and-fast generational differences besides whatever war was going on when a Neko was created. Some Neko, obviously, can transcend their generational gaps by being extremely fierce warriors or accomplished non-combat experts in their fields.
