In which episode is it revealed that Brandy is 15 years old?

Yes, you must be right. Now that I come to think of it, in "Bad Hare Day", Brandy says to Whiskers, “Didn’t you tell me you were 6 years old?” (or something along those lines; it’s been years since I saw it!), yet I too don't remember Brandy's age ever being revealed in the series. I must’ve gotten the idea in my head of her potentially being 15 years old from somewhere online. I imagine that in the show she’s the equivalent of a 16-17 year old teenage girl, but to be fair it doesn’t really matter in terms of the episodes. Thanks for clearing this up!