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

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.