Jack entered the reality in which puberty didn't start until age 18. This left an approximately 8-year gap in development. High schools now looked to be populated by 10-year-olds. College students looked like they belonged in middle school.
Because of this 8-year-gap, everyone over age 18 looks 8 years younger than normal. Jack noticed his beer belly recede considerably, and felt his hairline moving forward.
Jack walked down the street, and saw a girl who looked to be 15. He'd always wanted to have sex with a young body like that, and knowing that she was actually 23 made that possible. He walked towards the girl. "Hello, beautiful. My name's Jack. And yours?"
"I'm Dawn." She replied, smiling a bit.