Pages in different languages have no connection to one another outside the subject being linked. Each language’s page is written and moderated by speakers of that language who can choose to write whatever they like.
Wikipedia is apparently banned in China, so on that basis, I’d probably be a bit dubious about content regarding life there where I couldn’t verify the sources
It’s an analogy to “peak oil”, meaning the time at which something is at its highest level to-date, but will only reduce from here.
I suspect it’s probably not quite taking that meaning here, cos 18 year olds as a demographic peaked a long time ago, but with the context of the article, I suspect they’re mainly talking about 18 year olds choosing to go into higher education