User blog comment:Kaijusaurus-Rex/New TAWOG Season 3 Footage!/@comment-24472739-20150102143124/@comment-25624223-20150102201955

In this clip Anais says "That's girlest." and Gumball says "No. It's because you're four." Before she says "Then that's agest." -Meaning Anais is still four here.

Also, the whole point in The Kids is pretty much to show how Gumball and Darwin aren't aging. Ben Bocquelet already said that Gumball is staying 12. If he was turning 13, there would be no point in changing the voice actor.

And lastly, "Gumball is awesome" isn't exactly a birthday cake. There's the cake, the headphones, and Anais trying to see some sort of movie it seems. Seems more like she's trying to persuade him.