So if we go with the idea Shermy is older then both Stans…
in 1972 Stanley was kicked out Gravity Falls is set in 2012. Lets just assume they were seniors in high school so they would be 17-18 lets say 17 to be safe. 40 years have passed. Sounds about right when it states Stanley hadn’t seen his brother for 10 years then spends 30 to bring him back. 40+17= 57. Most GF pages state both Stans being early 60 so maybe its actually been more then 30 years? I’d have to be 33 which honestly at that point perhaps Stan lost track of time a little? Lets just stick with a solid 30 to try and stay as close to canon as possible.
Lets just guess that Shermy in 1972 is in college. There are so many options but the one I’m going to is he’s about 19-21. The child in Ma’s arms is HIS kid. The chances of knocking up a girl on college wouldn’t be too weird. if the kid is almost 1 then about 1 ½ has passed since he got the girl pregnant making him about 18-20 when it was conceived. We can just assume Ma is taking care of the child while the older one is in school.
There is also plenty of possibilities where Shermy could even be out of college at this point and just too busy working to take care of the kid. (The mother may or may not be in the picture) I just think him being a young adult at the child’s birth gives more reasoning for Ma to be caring for him. Though she could also just of been visiting with the child and the child could of just been there for the night.
This could also be why Filbrick is putting the weight on Stanford to make the money since the first kid just knocked someone up and left the kid with them.
Why Shermy isn’t mentioned in their childhood? Could of just been an older brother that didn’t want to play with ‘little kids’. A lot of times twins will stick together and ignore other siblings. Or the other siblings just are interested. I mean when they were 12 he would of been 16. aka angsty teen years.
IF Shermy is 21 in 1972 40 years later he would be 61 a normal age for a grandfather of 12 year olds. (A little young imo) It would also make Mr. Pines about 41 in 2012 making him 39 when the kids were born. (An average age to be having children)
(As long as Shermy did not marry or had Mr. Pines before getting married and didn’t bother to change his last name Shermy could 100% be a female. There is nothing that officially states its a brother. Though the Wiki states its a male I don’t think that’s official Gravity Falls info.)
TL;DR Shermy should be around 61 (perhaps a little older)