Re: "My old photos (pre-2013) are gone from my photos! I am running Sequoia 15.4.1."
Wondering if your Mac may be short on memory / storage space ...
eg: A Mac generally needs at least 50 Gb free in which to operate smoothly, and do all we ask of it.
(NB: Any space you have free in iCloud would be quite separate from the space available / required / on Mac.)
See used and available storage space on your Mac - Apple Support
If your Mac is short on storage space it is generally wise to back-up the Mac's data, before making more space available for use... eg: How to: Optimize storage space on your Mac - Apple Support
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Re: "Is there any way of accessing old iPhoto libraries?"
You may be able to recover the photos if you have enough space for them:
See what to do: If you're missing photos or videos in the Photos app - Apple Support
Else: You could try checking for any back-ups you have made, (eg: Time Machine, external or zip drive.)
Sometimes family membrs have copies, or you may have copies in old emails sent to others, etc.