Generation X, a synonym of millenial generation, is a pretty wide group of people, born between the early 1980's or the late 1970's (depending on the commentator) and the early 2000s.
I was never a big fan of those generalisations though. That would almost put me in the same generation as my mother.
I tend to think of people as "of my generation" if they weren't born, or were too young, to take part in the strange fashions of the 1980s and the early 1990's. Someone who listened to the b... unique songs of the 80's willingly, contemporarily and as an adult is definitely not of my generation. Or if they lived long in the period when it was okay to smoke everywhere and all the time. Or if they were adults when they first used the Internet.
Though it makes me feel old to say that those who were born with computers roughly in their modern state (say, 2000-2004 and later) are probably no longer of my generation. Because mentally, on many points, it feels like I never got past 12 years old.