Even as Dipper gets older Bill still stands by her side not aging since he's immortal. Dipper like 80.

Mystery shack

"Hey Gram-Gram hey Poppy!" Their grandchild Abby and Stan shouted running out their car to their grandparents and hugged them.

"Hi Abby! Hi Stan!" Bill said lifting them up "Aren't you too getting older!" He said putting them down. Bill and Dipper were on the porch chairs

"Me and Stan got a puppy named Sammy!" Abby said jumping up onto Dipper's lap who immediately recoiled putting her down.

"Yeah for our birthday!" Stan said. Stan and Abby were twins and 5 years old Both had blond but Abby had green eyes while Stan had gold eyes.

"That's nice kids." Dipper said trying to get up but has some difficulties "Bill honey can you help me?" She asked quietly.

"Of course Pinetree." Bill said helping his wife up since he had demon strength and aging doesn't affect him at all meanwhile Dipper is slowly going down by age.

"Mom!" Bill Jr or Billy yelled running up to Dipper "Are you okay?" He asked worried.

"Just what getting old gives you am I right?" Dipper said chuckling "Well I'll go make dinner." Dipper said shuffling inside.

"Dad why does Gram-Gram need help getting up?" Abby asked.

"Cause she's getting older." Billy said sitting down putting his twins on his lap.

"But Poppy doesn't need help and he's old." Stan said pointing at Bill who looks old too.

"I'm a demon kids I may look older but that's only because I chose to so me and Pinetree can grow old together." Bill said putting his fingers between his hair "But I did jeep the hair." He added smiling.

"Kids! Dinner!" Dipper said so Stan and Abby walked inside with Dipper.

"Just out of curiosity do you and mom still you know?" Billy asked.

"Oh you know it!" Bill laughed patting his sons back who has a shocked look on his face.