I suspect the numbers would be worse if you looked at households instead of individuals due to declining marriage rates (but I'm not willing to put in the effort to find numbers).
I don't understand what you're saying. To me, if the rate stays flat but represents fewer married couple, then this same rate actually means more homes are owned by people of that age.
Meaning - if guy and girl are married and own a house together, that counts as 2 people towards home owner bucket.
If they are not married, they'd need to each own a house for the same rate to hold.
So I never dug into this before but I found this interesting chart from the US government [1]
It does seem like a relatively smaller percentage of young people own homes today vs 40 years ago - but not as shocking as you'd imply:
Percentage of people <35 who own a home: 1982: 41% 2021: 38.5%
Percentage of 35-40 who own a home: 1982: 70% 2021: 62%
So while the trend is down - it still seems like over a third of people under 35 own a home, and over 60% of those 35-40 do.
That sounds very far from impossible.
[1] https://www.census.gov/housing/hvs/data/charts/fig07.pdf