Well, apparently the Pew Research Center has announced the official definition of a millennial:
Anyone born between 1981 and 1996. So anyone 22 to 37 this year.
That means millennials were between age 5 and 20 on 9/11, ages 12 to 27 when Obama was elected, and ages seven to 22 when Sammy Hagar temporarily reunited with Van Halen.
Here's Pew's full breakdown of the generations:
1. Silent generation: Born 1928 to 1945
2. Baby boomers: Born 1946 to 1964
3. Generation X: Born 1965 to 1980
4. Millennials: Born 1981 to 1996
5. And for anyone born after 1996... well, there's no official name for you guys yet.
From Pew Research Center - Defining generations: Where Millennials end and post-Millennials begin