Does reposting new account's content on a much older account have a positive effect?

An older account may help, but it does not always mean better results. It depends on the account history, audience and how well the content fits the topic. I would focus on sharing useful and relevant content rather than the account age.

Yeah, it helps a lot if the old account still gets decent impressions.

It basically pushes quick initial views and profile visits to the new post, which gives the algorithm the green light to push it further.

Just use the Collab feature instead of a story repost if you can. Works way better since it merges the reach and engagement on both accounts.

yeah, from story section you can be get some reach or impression. but from repost section i don't think so you can gain any views or reach. you can try this to know the result.

story mention works well here and collab posts can also bring both pages into the same reach pool repost it straight onto the old feed