I have variously seen the upper boundary for Gen X as 1976, 1979, and 1982. Having been born toward the end of 1980 and comparing myself to those born in 1970 and 1990 (squarely in the middle of their respective generations), I find myself having far more in common with the latter. Hence why I consider myself a millennial.
The Strauss & Howe definition, which I find most useful for cultural purposes, puts the line between '81 and '82.
With that said, it's been my observation that people born within a few years of the line in each direction -- as far as '77 to '88 -- tend to share the same values and have characteristics of both Millenials and Xers (born in '84 here, and I'll gladly apply that description to myself). I've also seen people, particularly those in the S&H community, use the term "Generation Y" specifically to refer to this cusp and not to Millennials as a whole.
Gen-X stereotypes tend to be more true for people born '76 and earlier, and Millennial stereotypes tend to be more true for people born '89 and later.
There are probably other things that play into the stereotypes but being born '89 and later maps pretty well to never really knowing a pre-Web and pre-Mobile world. A somewhat primitive version of it in their tween/early teen years to be sure but an online world nonetheless.
But never will I give up the skinny jeans!