My oldest rental property was built in 1975 (newest in 2007); however, it also matters if you are doing a rehab on the property to make it rental ready first. Because after rehab it may then have a new HVAC, Water Heater and Roof, plus Electrical and Plumbing have likely been checked or re-done. So if all the systems that commonly breaks in a rental are brand new, then the age of the structure itself may not be as important. Most of the rentals I buy are in need of rehab before they are rental ready, and I fix everything from the start or make a plan to do so when needed.