I think the point parent is trying to make is that the majority of postdocs will move to an industry job simply because that's where most of the jobs are, even if they'd prefer to remain in academia.
While postdoctoral jobs aren't age limited, they are defined in the US as a temporary training position, and it's not possible to remain a postdoc for very long. The vast majority of postdocs in the US are foreign nationals, on a visa scheme that limits their stay to a theoretical maximum of 5 years. The salary is low enough that it dissuades postdocs from remaining in these positions for very long anyway. Some institutions may support transitioning senior postdocs to a more semi-permanent "associate" position, but that's not usually the case. The visa scheme also disallows postdocs from applying for a green card (while maintaining status), which complicates matters further.
While postdoctoral jobs aren't age limited, they are defined in the US as a temporary training position, and it's not possible to remain a postdoc for very long. The vast majority of postdocs in the US are foreign nationals, on a visa scheme that limits their stay to a theoretical maximum of 5 years. The salary is low enough that it dissuades postdocs from remaining in these positions for very long anyway. Some institutions may support transitioning senior postdocs to a more semi-permanent "associate" position, but that's not usually the case. The visa scheme also disallows postdocs from applying for a green card (while maintaining status), which complicates matters further.