As humans live longer, the precise modeling of mortality curves in very old age is becoming more important in aging research and public health. Here, we address a methodology that utilizes a modified stretched exponential survival function where a stretched exponent is relevant to heterogeneity in human populations. This function allows better estimation of the maximum human lifespan by providing a good description of the mortality curves in very old age. Demographic analysis of Swedish females over three recent decades revealed an important trend: the maximum human lifespan (existing around 125 years) gradually decreased at a constant rate of ~1.6 years per decade, while the characteristic life gradually increased at a constant rate of ~1.2 years per decade. This trend indicates that the number of aging people is increasingly concentrated at very old age, which is consistent with the definition of population aging. Importantly analyzing the stretched exponents would help in evaluating the heterogeneity trends in human populations.