At What Point is a House Too Old

[Ernest Grindle]

Hello BP! In looking at buy and hold properties, I find a lot of old 1900's homes available that including repairs still have good cash flow. But I'm wondering at what point is a house too old. If I invest in rehabbing these older houses, should I expect the house to last another 100 years with the proper maintenance, or should I consider a house's life expectancy to always be on a downward trend because maintaining it is no longer cost-effective?