He keeps saying that he isn't a kitten anymore and his brother is three years younger and has gone to boarding school which I think happens when u go to highschool especially in rich families so I think he might be older than 16, it's also mentioned that his body is smaller than what an average cat's his age is so he could be almost 18 I think assuming his brother is in highschool around 9 or 10th grade at most. So he could range from 17 - 20 years old if the story doesn't follow the way of 'cat years' and follows average 'human years'
Even if they followed 'cat years' he'd been relatively old. Idk this is my guess. He could also be a late bloomer aswell given how different his body is to average cats
how old is this cat dude,, he looks suspiciously young...