The accident killed Max and Ruby's parents, and left Max with a head injury, affecting his ability to talk. It's also the reason why Max is always playing with ambulance and police car toys.

Why is Max so silent?

"Max is mute because their grandmother molested him at a young age and he is so traumatized that he created Super Bunny as an imaginary hero to somehow save him from his life. He also plays with the ambulance and police cars because he remembers those vehicles were there during the accident."

When did Max start talking?

In seasons 6-7, Max is old enough to go to preschool and changes his style from blue overalls to a white shirt with red stripes and blue jeans. Max always spoke in one word but sometimes could talk in full sentences, but in Season 6, he begins to speak in full sentences.