If the age restriction is implemented entirely via the Patreon plugin, then the plugin description does talk only about support for posts rather than pages, suggesting that it doesn’t support pages.
But I found a post from a user at wordpress.org/support/topic/no-support-for-pages/ which was subsequently updated to say “OK, pages were added so, I now give this plugin five stars”.
Sadly, it gives no clue about how to make it work on pages.
But DuckDuckGo thinks it knows the answer:
To restrict access to pages using the Patreon plugin, you can find the ‘Patreon level’ metabox when editing the page and select the appropriate tier from the dropdown menu. This will lock the page for patrons at that tier, allowing only them to access it.