2x23 - The Grim Adventures of Billy and Mandy

After missing on a school trip, Grim takes Billy, Mandy and Irwin to the "Halls of Time" for a tour. There, Billy and Irwin mess the hourglasses that symbolize each person's lifespan. When they leave, they accidentally leave their own hourglasses, including Grim's, upside down, resulting in the group getting younger as time passes. Grim allows the kids to disappear by going before birth, but he cannot restore his own hourglass to its correct position and he disappears too.