I'm gonna eat you if you don't stop saying sorry, Snowi.

In general how fast or slow your Narakari are aging is your decision. Anything else doesn't seem to be fine for me because people have different real lifes and spend different amounts of time with games or hobbies. And if they would age in real life time foals won't be that much fun because their owners would have to wait years until they can "use" them. O.O
The only thing that should count for the Narakari is the time within the story or whatever it is used in. We give them out with a particular age and this will be the age of the Narakari when it "enters" the storyline.
And you don't have to "reset" its age when you finish one storyline and it might be handed over to another character. As nobody knows how old they become it seems to be possible that they outlast one owner. But this is again your decision. Sometimes a reset might be better, another time it won't fit...