There is rebake old posts count option in admin settings with this description:
Number of old posts to be rebaked every 15 minutes
At first I thought that Discourse always rebakes some old posts, but after looking at the
baked_at field of old posts I noticed that it does not happen.
So I looked into the source code, and it seems like this option is used only for rebakes after some major updates/fixes in Discourse (and so far there was only one such case in 2017).
Maybe the description should be improved.