I’d say it extends back to the 70s. The 90s saw a little reversal but it was short lived. The long term trend seems to start in the 70s and continues to this day.
It started in the 1930s, and would have gone on continuously except it was interrupted by World War II. So there was a brief anomaly from the 1940s through the 1960s where Americans were able to enjoy a delusional standard of living because we were the only major country in the world with our industrial base and infrastructure unscathed by the war.
My mother said she watched their income pay for less and less even as it rose through promotions and pay raises from the 1970s on.
It is tricky to sort out these things. I don’t know that millennial and Gen Z have a harder time getting jobs as I (late Boom generation) can’t even get an interview in the same area my millennial children get jobs easily....
But my generation was not saddled with college debt, which is a huge difference.
Another difference is that from the mid-Boomer generation on, everyone was encouraged to follow their dreams. I have totally changed my mind on that: think of something that will not drive you crazy if you do it for 40 years and then go for it. Passions, dreams, and parachutes are for free time not paid time.