They were toying with the idea that in 2015 it will be normal for chavs to have their 1st child younger then them.
So, anytime after the 1st film is set, lets say 1986 Biff has his 1st child. When that child is about 12/13 (1999/2000), along comes Griff:D
Notice though in BTTF3 at the end...