Tanner wanted to have something in common with his brother, but he didn't want to be the shrimp he was when he was younger, also he didn't want to take the risk to make Derrick older, in both situations Derrick would try to take control.
So Tanner took a look on Chronivac for other choices, and he found "Mental Age" selection, he could be mentally younger so both they would have something in common and he would keep the control.
And since they didn't have to go out, there was no risk of embarass himself.
He chose 12 for the mental age and 15 for the physical age, he didn't want to have a huge gap between mental and physical ages. He set it so he would go back to normal sunday night, and the changes started.
Physically, his height dropped from 5'11" (180 cm) to 5'6" (168 cm), most of his body hair receeded back into his body, but he still had slight wisps. His facial features became younger.
But the most important changes were mentally, he became more immature and he felt an energy wave going inside his brain. He felt the urge to go out and play, also he forgot everything he learned in school in last 5 years.
Derrick, who was now back at his normal height, asked what was going on.
"Can't you see silly? I'm younger so we can play together!" said Tanner, with a cracking and immature tone of voice.