My best friend just thought of something:
if Dip and Mabel’s grandfather was a baby when the Stans were teens moving out, how did Dipper and Mabel’s parents become close enough to Stanley (or Stanford as far as they know) to trust him enough to send their children to stay with him for an entire Summer?
She also pointed out that if their grandfather was a baby in the late 60s, then both him and his son would have been having kids in their teens.