The Pew Research Center has enough authority that their opinion matters on stuff like this. They just 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, there's no official name yet for the generation.