I did a few calculations and I figured out that Stan's line in Dipper and Mabel vs. the Future about being in his 60s is incorrect. If you look at the calender in Carpet Diem it says 1982, this is implied to be the year Ford got sucked into the portal. Since we know that this happened 10 years after Stan was kicked out, we know that the twins were 17 or 18 in 1972. This would make them 57/58 in the time of the show since it takes place in 2012.