As Gen-X, I can confirm that 30 is far more traumatic than 40.

I totally agree.

When turning 40, I had much to much going on (family stuff really taking up speed, buying own flat,...) that I didn't give a shit.

30 on the other hand, with me still without a long-term partnership and just continuing living my old student bachelor's life, felt like a huge thing, triggering profound eleventh-hour-panic.

I think that the dates have shifted as typical family foundation has moved from the late teens or early twenties to around 30.
This also means that 50 might be the new 40 (A fact that I can confirm by now), marking a new phase in life as 40 maybe did in former times.