I think pre-internet here means more… pre-omnipresence-of-the-internet? Pre-social media, especially. Pre-everyone in middle school had smartphones? That definitely strikes a chord with me.
I’m 23, and just graduated from college in May (same as Natalie), so I’m riiiiiiiight on the edge of this too. I consider myself a millennial because I think my worldview matches up more with the one I associate with millennials, but my sister, who’s four years younger, definitely is more Gen Z. And for me, the difference has almost felt more like a difference in attitude towards the world.
This may be something I’m basing too much on my own feelings, but I feel like the worldview people associate with millennials comes from having our worldview defined by the 2008 recession (which I think is in line with what @Navster said), as the oldest end of the generation (early-80s) was starting out in the workforce and the youngest end (late-90s) was old enough to understand what was happening when their parents lost their jobs or their lifestyle suddenly changed. For me, that felt like it imbued a lot of cynicism and frugality (that maybe is shared by Gen Z), but also a feeling of… not helplessness, not exactly, but maybe heavy pessimism? Both a heavy distrust in systems and a sense of the inevitability of those systems’ failure? I work with a lot of high schoolers in my job, and while I think they share that cynicism to an extent, I don’t think they feel nearly as pessimistic about the future as I and my friends (who’ve just graduated) do. Or maybe it’s more that they feel they can change it more readily.
(Or maybe that’s all just me psychoanalyzing myself haha. But it’s a thought.)