Chinese economy to overtake US 'by 2028' due to Covid
bbc.com
bbc.com
China has similar demographics, but is also larger. It has roughly 4X the number of people as the USA, so all it would take to equal the USA in aggregate is reaching 1/4 of US GDP per capita. GDP/capita is a more meaningful measure anyway.
https://www.populationpyramid.net/china/ Here is a neat visualization of their demographic situation. There are worse demographics but the point is that they have run out of labor for an export led economy and the internal base for a consumption led economy. Same thing is true for Germany which means they will have to find another production model which means major political instability.
Edit: Or coal shortage.
Their own predictions from 4.5 years back have a gdp slump after brexit, even if we stayed in the single market (which we're not AFAIK).
https://cebr.com/reports/the-economics-of-the-eu-referendum/