I hunted for my question and found an interesting Economics Explained video on youtube. That hypothesizes that Baby Boomers essentially voted to change policies to favor themselves as a cohort.
The situation is even more extreme in places like Italy where the young are even less numerous compared to the old than in the UK.
It's the political equivalent to deflation. If every generation is smaller than the last, you end up with a gerontocracy and no-change society. To maintain pro-growth policies, you need a critical mass of young people who embrace change and want to grow. Otherwise you get stuck in a positive feedback loop of more and more power and money concentrated among the old which depresses the birthrate.
We desperately need a pro growth government next. Desperately.