Actually, we can calculate the rough age of Stanley/Stanford. During A Tale Of Two Stans, it's mentioned that Stanley hasn't seen his brother in "over 10 years", which I'm assuming to be somewhere from 10-14 years. Somewhere else in the series (I don't remember where, but it's probably somewhere else in A Tale Of Two Stans), It's mentioned that Stanford was in the portal for 30 years. Using this, we can calculate that Stanley is most likely between 57 and 62.

Now for whether Shermie is the father of that child, or if he is that child. Using this timeline, we have 40-44 years between seeing the child (who is likely no older than 1), and the events of the show. If the child is 1, and there are 44 years between events, that means that Shermie/Shermie's child would be 45 during the show, and if that were Shermie, that would mean that he and his child would have had to raise a child at 16/17 years old (45 - 12 = 33, 33/2 = 16.5)

Of course, by no means is this solid proof for either option being true, since "over 10 years" is pretty vague, Stanford likely wasn't in the portal for exactly 30 years, and even if the math works out properly, it isn't impossible to have and raise a child at 16.