Just A Ten Minute Walk In The Woods

In this story, Dipper goes for a walk and ends up breaking the Bill statue. Bill then sends them both to the Nightmare Realm and fuses his power with Dipper. Dipper ends up traveling the multiverse for ten years with a group of outlaws, and even gets married. One day, Mabel accidentally reads out a spell that brings him back. The question is, why is Dipper the only one who's aged?