Well actually they don’t turn 20 tomorrow.

I did some math and the show ran for 3 years and 8 months which equaled only from June 1st-August 31st. So assuming 3 years and 8 months is the equivalent of only 3 months for them I just took both number and multiplied them by 4 to get that 1 year for them is equal to 14 years for us.

But I also have to take into consideration that they are only in Gravity Falls for the summer which means that possibly 1 year of us is only 4 years 5 months for them, to get this number i just added 9 months (the time they arent in Gravity Falls throughout the year) and added it to the 3 years and 8 months.

Now is where I figure out their birth years and the years they turn around 18 for both of these scenarios. Ok lets start with the top one shall we. If we take 14 and multiply is by 13 (their age at the end of the show) then we get 182 this is how many years we have to subtract to find their birth year which (reminder im subtracting this from 2016 since thats the end year in the show) (never mind i’ll actually be subtracting this from 2012 since I just realized that technically this is their 2012 summer) ok so anyways if we subtract 182 from 2012 we get the year 1830 for their birth year. Now for the year they turn 18, lets do the same thing shall we but instead multiply 14 by 5 since we are simply just adding 5 years to them normal time which is 70. So now we just add 70 to 2012 and bada bing bada boom they will turn 18 in 2082

Ok now for the 4 years 5 months one. Imma just do the same math for each but this time will have to be different since 4 years and 5 months time 13 isn’t an equal amount of years i will just round up or down accordingly when we do this we get about 57.4 years so 57-58 years. Now we just subtract this from 2012 and we get their birth year as 1955-1954 (edit: I now realized that it actually would stay as 1999 since this is their first time in GF so every year prior would be equal to us) Now we do it for when they turn 18 which is 22.08 so 22 years is what we add so they will turn 18 in the year 2036 not as long a wait as the other way.

TL;DR Dipper and Mabel were born in 1999 or 1830 and will turn 18 in either 2072 or 2036 thank u for coming to my Ted Talk