perhaps it's time autorenew was deprecated in favour of allowing autoreplace to the same vehicle?
I had similar toughs (however without 'only old' constrains).
Autorenew option is buried deeply and is not intuitive that there is different method for same vehicle than for changing vehicles.
One note: autoreplace should automatically pick same vehicle, if no new vehicle type is defined, and drop it, if same vehicle is no more available.
My experimental openTTD server: 220.127.116.11:3979 non-standard client, now testing: JGRPP http://tiny.pl/ggnch
Projects: Reproducible Map Generation patch
, NewGRFs: Manpower industries, PolTrams, Polroad, 600mm narrow gauge, preindustrial houses, wired, ECS industry extension.