how old is Rod?

The movie doesn't specify, but he has already spent at least a year in college at the start of the summer, where Eric has at least one final year of high school left. If Eric is on the edge of seventeen at that point, Rod would presumably be between 18 and 20.

The movie doesn't specify, but he has already spent at least a year in college at the start of the summer, where Eric has at least one final year of high school left. If Eric is on the edge of seventeen at that point, Rod would presumably be between 18 and 20.