Final answer:
The age life method of estimating depreciation is centered on C) The age of the building and its expected remaining lifespan, as it allocates the cost of an asset over its practical service life based on its wear and tear.
Step-by-step explanation:
The age life method of estimating depreciation is a technique used in accounting to allocate the cost of a tangible asset over its useful life. According to this method, the primary factor used in calculating depreciation is C) The age of the building and its expected remaining lifespan. The reasoning behind this method is that the physical deterioration and obsolescence of an asset are somewhat predictable over time. Therefore, depreciation is determined by considering how old an asset is and how many years of service it likely has left.
This approach contrasts with other methods, such as those based on an asset's original cost or its current market value. Instead of reflecting market conditions or investment size, the age life method focuses on the realistic expected usage period of the asset, thus it's more about the wear and tear and less about external economic factors that can affect other depreciation methods.