The expires header is related to the page, not fragments of it.
Lets go over the basics.
You send HTML to the browser, all of it or a stream, doesn't matter for the example.
The browser receives it and starts parsing it. At the same time, once it starts parsing, all the other linked files are called, like images, css, js, etc. Some parallel connections are established and some of the linked files start coming; the browser detects them and assigns the right parser/renderer to work with the file.
If the file is an image it will be displayed on the screen. if it is a CSS, the parser starts applying the rules to the received HTML and shows/modifies the presentation of the content. If it is a script, it reads it looking for a trigger, if there is none, the file just stays on memory, if there is a trigger, the called action/function starts working.
Above the fold is a term that refers to the content that is viewed first on the screen, it comes from the news papers.
CSS and JS are not above the fold, they are asynchronous or parallel to the rendering of the content; CSS affects the whole document, JS affects whatever it was programmed to do.
If what you want to do is control the cache of the CSS and JS files, you can use rules on your .htaccess, like so
Header set Cache-Control "max-age=31536000, public, s-maxage=31536000"
Header set Pragma "cache"
or like this
ExpiresDefault "access plus 1 month"
ExpiresByType text/css "access plus 1 month"
it is not possible to affect part of the content of a page, unless that content (HTML file) is inserted as an iframe; in that situation, you are still affecting a different file, but the final page, may look like only part is affected by cache rules.