I always wonder about YouTube’s storage infrastructure and whether they move old videos to cold storage in a way that is costly to recover, and that maybe that’s why they try to discourage watching older videos… but that is probably a bit oversimplified.

Anecdotally, there must be some difference, as it often takes longer to get a video started if it's old.

This has always been my suspicion, considering older videos (or just videos with not a lot of views in general) tend to take noticeably longer to load.

My immediate assumption is these videos are not edge cached, so naturally they want their platform to avoid delivering these types of videos. They would rather stick to the well-cached, proven popular videos that are currently circulating.