The basic thought process is that buildings are meant to have a useful life of 100 years, and so most estimating systems use this as the criteria for depreciation.
The biggest mistake ever made on depreciation is that it is NOT age alone that forms the basis for depreciation, it is AGE & CONDITION which when properly applied and assessed creates the proper applicable depreciation.
For instance, I live in a home that was constructed in 1750 and has been constantly up kept and improved. Using most systems I would have to depreciate my 20" thick stone walled farmhouse 258% to be in line. Well that dog don’t hunt where i live.
Woe be gone the individual that applies that amount of depreciation in this case, glad I have dobe's................................