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 2018 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
In applying the concepts of economic life, effective age, and remaining economic life expectancy, the appraiser considers all elements of depreciation in one calculation. Therefore, the effective age estimate includes not only physical wear and tear but also any loss in value for functional and external considerations. 
— The Appraisal of Real Estate - 14th Edition, page 599