While drinking a slushy, Cosmo claims he can't get brain freeze, thus proving he has no brain. However, in a previous episode while disguised as pistachio ice cream, Cosmo licked himself and turned blue with brain freeze.
There was an episode where Timmy was going to have a birthday the next day. He's normally 10 years old, so we assume that he's going to turn 11, but the episode never mentions his age, therefore. It's possible that the directors decided to make him nine prior to the birthday.