According to the 1979 Rolling Stone article about the band, Glenn graduated in 1966, which would've made him 17. I guess that the "you must be born before September 1st" rule didn't exist then.

From the article
"Following graduation in 1966, Nellie wanted to send her son away to college. Glenn wanted to stay in the Detroit area and play music. They compromised at community college near home."

Here's the whole article in case anyone wants to read it:

http://www.rollingstone.com/music/ne...9791129?page=5