Economic Life

The period over which improvements to real property contribute to property value.
— The Dictionary of Real Estate Appraisal - Fourth Edition, page 92

National Building Cost 2017 assumes the following Economic Life values:

Q1 though Q3:                       70 Years

Q4 and Q5:                            60 Years

Q6:                                         55 Years

Age-Life Method

A method of estimating depreciation in which the ratio between the effective age of a building and its total economic life is applied to the curent cost of the improvements to obtain a lump-sum deduction; also known as the economic age-life method
— The Dictionary of Real Estate Appraisal - Fourth Edition, page 8

Depreciation

a loss in property value from any cause; the difference between the cost of an improvement on the effective date of the appraisal and the market value of an improvement on the same date
— The Dictionary of Real Estate Appraisal - Fourth Edition, page 79 - 80