I think the "7 year theory" originated from the 7 year chakra cycle.
In TCM women also have a year cycle.
Either way, as you pointed out does not transfer medically.
With that said, I think its the human flaw of wanting a new beginning that is appealing and keeps the myth going.

What this post does highlight well is that the body CAN regenerate.
This is also seen clearly during injury (not just regular turnover).

Our natural state is health!