A character string representing an age interval (given by the starting age of the interval).
1950-1955, 1955-1960, …
mx for the given five-year time period (i.e., from 1 July in year t to 1 July in year t+5 such as the period 1950-1955 refers to the period 1950.5-1955.5 and the mid of the period is 1953.0).