I'd say it really depends on the vehicle, and how well it is aging.
My 2010 Fusion, for example, with 272,500 miles on it, still gets 7,500 mile OCI's. UOAs show that this is still working well.
A vehicle in worse condition, or one exhibiting issues, might require a shorter OCI.. but IMHO, it's really a case-by-case thing. You can't make a blanket statement strictly based on the vehicle "getting older"