I think 1981 - 1996 seems like a good estimation. You have the people who saw the internet as the "big new thing" when they were leaving highschool/going into college, to the people who witnessed smartphones become the big new thing while they were leaving high school/going to college. There's also the factor that like, the rise of the internet isn't the only thing that defines the millenial generation. I'd argue experiencing the great recession before your 30's is something that defines the millenial generation.
I really can't see calling someone in 1985 a gen x'er. My sister is 12 years older than me and was born in 1974. In highschool, she could take a typewriter class as an elective, and there were I believe several levels of it, so you could take more than one semester. I feel like maybe someone born in 1980 might have experienced that, maybe, but I really don't think anyone (in my city anyway) born in 1985 was learning how to operate a typewriter in highschool.