So the cat guy has fake ears so he is also from the human world?
And is it so hard to talk to an mage or a teacher and explain that you are from an extremely short lived race (avg livespan 75y) of Humans, so you already lived 1/3 of your life and already learned lots of stuff, minus magic which don't exist where you live. You can prove it by doing advanced mathematics.
As for children saying they are adults, it's one thing if a childish child says this, but totally different when a toddler says "this human form is so limiting".
As for adv math problems (there may be races which specialize in math as they are part robot/golem/dungeon_core, but), there are plenty of ways to prove that you are cognitively and socially mature. It is not the knowledge, but a way of expressing it that would prove his point.
Using magic to prove this is also ok. He would just ask them to provide a truth-telling spell or artefact and use it saying he's adult, but from a foreign short-lived race and somebody used magic without his consent to kidnap him here...
(I don't know how to reply (there's no reply button, just Report), so I edit my post. Sorry)