That's millennial slang for "I was too young (not born, yet) to have been there the first time around."
That's really it in a nutshell. The 70s are what the 50s and 60s used to be. Anyone who remembers the 60s as an adult is.... well..... not a spring chicken. Those people are carrying 1 pound heads, three pound cans, and 7 pound basses to the gig. And they don't like anything 70s because it came out after they were in high school (which is, evidently, the most important time period in the history of the planet).
But kids today see the 70s as way back then.
I told someone the other day that it won't be long before we refer to that era as "the 1900s" kind of like we do "the 1800s" and "the 1700s" when referring to history.
The English language didn't stop evolving when baby boomers became high school seniors. It's usually changed by the generation that is currently 18-30 years old.
Short version: "That 70s thing" = "old school"... unless you're old.... like really old.
