Board Thread:General Discussion/@comment-2207994-20141010234931/@comment-25094659-20141114220112

Thel Vadam4321 wrote: They would be second cousins to Mabel and Dipper. They would probably be in their early twenties or their thirties. They wouldn't be in the same age group.

Even then, I don't think that Stan is necessarily a father. If Stan had grandchildren they mighy be around Dipper and Mabel's age, since he's their granfather's brother.