I thought my (Millennials) generation was born in 1992 or afterwards. So I was about to challenge your 'mid thirties' assertion. Wikipedia also has a wide range of start and stop birth years, ranging from 1976 to 1996 as the start birth year. https://en.wikipedia.org/wiki/Millennials#Date_and_age_range...
I'm riiight on the cusp; born in '82, graduated in 2000. My wife was born in '81, graduated in '99. We're definitely more millenial than GenX - my older siblings (born in '70, '72, '75) are total GenX'ers. The GenX'ers had computers as tweens/teens, so they definitely grew up as all this tech stuff was developing, but I've literally never lived in a house without a computer. Quite literally: my parents bought a ZX-81 for my older siblings the same week I was born.
We both have younger siblings who were born in '84 (graduated in high school 2002). They're the oldest of what I'd call "true millenials" - I think one of the big distinguishing things is that those who graduated after 2000 had Facebook in college, whereas I didn't. Sure, there was ICQ, IRC, various web forums, Myspace and Friendster and all that stuff, but it wasn't quite the same; those didn't have nearly the reach of Facebook, and most of the people I interacted with pre-facebook I didn't know IRL. I used Facebook to keep in touch with College friends after we all graduated and did the whole post-college-diaspora thing, but my brother and his friends used Facebook constantly as a key part of their social life.
Additionally, cell phones were much more prevalent for the "true millenials" - my brother and I both got cell phones in '03, and for him it was a core part of his college experience.
Facebook as a determiner is an interesting one. I'm also on the cusp but delayed uni by a few years, and so Facebook popped up while I was at uni and changed everything.
That's pretty much exactly what I said; those graduating HS after 2000 didn't have FB in college. I graduated high school in 2000, college in '04, didn't have facebook till after graduation. My brother graduated HS in 2002, college in 2006, facebooked all the time.
Imo, there's a sandwich date range from around 1976 to 1982 where you either identify as Gen X or Gen Y/millennial.
I'm someone in that gap and identify much more closely with millennials than with the Gen X slackers, though a lot of my childhood friends also in that gap identify more with Gen X.
Perhaps it's a combination of my job, that i feel i grew up with (along with?) the internet because my parents were pretty fast in getting connected, and that i have older siblings who are very much Gen X as in leaving college during very difficult job times in the early 90s, and I never shared their experiences.