We might think of it this way. While we were in school, we were contemporaries with children of other ages who were also going through school. Even though our ages differed, we were collectively known as 'school age children.' We were all part of the same generation of students. Obviously the older ones finished their course, even while the younger ones were just starting out.
This is a stupid ilustration. They compare an 8th grade pupil and a 1st grader (7 years is the max difference) with an old person who was born before 1914 and a child that was born when the old person was 90 years old, for example (difference is 90 years). And they say they are the same generation because they "overlapped"! I'm the same generation as my grandpa because he was not yet dead when I was born! This is outrageous!
I was contemporary with grandpa's death, but not with him. He was born in 1922 and fought in WW2, 40 years before I was born.