Anyone know what the age difference is supposed to be between Gene and Nubsib?

Gene is 25 turning 26 this year (He literally says “I am ALMOST 26” which people seem to be misunderstanding) and Nubsib is 20 turning 21. Their age difference is 5 years in the show and in the novel.

In the novel, Gene goes to live in a dorm during high school. So we can assume that him and Nubsib met at the ages of 7 and 12, 2 years prior to Gene leaving for school. When Gene left they were 9 and 14.