Experience with assets which have attained an exceptional or unusual age shall, with respect to similar assets, be disregarded in determining the maximum expected useful life of the longest lived asset in a multiple asset account. For example, if a manufacturer establishes a proper multiple asset account for 50 assets which are expected to have an average life of 30 years but which will remain useful to him for varying periods between 20 and 40 years, the maximum expected useful life will be 40 years, even though an occasional asset of this kind may last 60 years.
26 C.F.R. §1.167(a)-8