Okay...so after watching the 2nd Goofy movie, I decided to try to calculate a timeline of Goofy's life...
In 2000, Max is 18 (going by the fact he is starting college and the movie was released in 2000). So this means Max was born in 1982.
Goofy mentions that he loved living during the 70s which means he has to be a preteen-through-teenager during that era.
Now, Goofy also has only 3 years of college experience. Using this as a guideline to him being a single father currently here is what I am concluding:
Goofy was born in 1961. He started college in 1979 and drop out in 1982 after Max is born. While his girlfriend takes care of Max he ends up getting random jobs to pay for his new family (As seen in the Goofy Shorts). Some time between 1982-1989 (In the Goofy shorts, Max is never really shown to be older than 7 years old). Goofy's wife/girlfriend passes away.
It is during this time period in which GOOF TROOP starts when Max is going through the ages of 10-14. At the start of the Goofy Movie, Max is near the end of High school and after High School it leads into Extremly Goofy Movie.
Using all of this information....we learn that in this time period, Goofy is ONLY 39 years old by the end of the Extremly Goofy Movie.