Millennials are those that became adults on or relatively close to the turn of the century, hence the name. If you played Oregon Trail in school, that's what you are. The next youngest is Gen-Z (Gen-X's kids), with the birth year starting at 1995. After that comes Alpha, the children of the millennials (being a millennial myself, that feels strange to say now. We gettin' old and sometimes I still don't feel like a responsible "adult". I still play video games at 41 for damn sakes. This shit's gettin' wierd.)

There's an approximately 15 year gap between the start of each generation.