Abstract: Real GDP growth rate in developed countries is found to be a sum of two terms. The first term is the reciprocal value of the duration of the period of mean income growth with work experience, Tcr. The current value of Tcr in the USA is 40 years. The second term is inherently related to population and defined by the relative change in the number of people with a specific age (9 years in the USA), (1/2)*dN9(t)/N9(t), where N9(t) is the number of 9-year-olds at time t. The Tcr grows as the square root of real GDP per capita.