If it is food related then at approx 4 months old I put mine on adult food from then on as kitten food is very rich for their tums!
Kitten's will have their adult teeth by 7 months old!
Now if its antics you are wondering about well....I have a two and nearly half year old curtain swinger, I have a one and a half year old feet in the bed grabber.....
My Pippa (dont know age) but she has just taught at least 4 of my cats how to pull the rubber sealant away from the sides of the double glazing!
I would'nt change any of them though as I loves my quirky lot!