Earlier we identified an Immune Risk Profile (IRP) of very old individuals, 86-94 years of age, characterised by an inverted CD4/CD8 ratio and associated with persistent cytomegalovirus infection and an increase in the numbers of CD3+CD8+CD28- cells. In the present study we included data from a population-based sample in the age range of 20-79 years to examine the prevalence of individuals with an inverted CD4/CD8 ratio relative to age and gender across the entire adult lifespan. Immunological monitoring that was conducted included analysis of the numbers of T-cells in the subsets CD3+, CD3+CD4+, and CD3+CD8+ as well as CD3+CD8+CD28+, CD3+CD8+CD28-, and CD8+CD45RA+CCR7+. There was found to be a significant lowering of the numbers of CD3+, CD3+CD4+, and CD3+CD8+, and of the CD8+CD45RA+CCR7+ cells across the adult life-span. Notably, the prevalence of individuals with an inverted CD4/CD8 ratio increased from about 8% in the age range of 20-59 years to about 16% in the age range of 60-94 years. The mortality rate in individuals with an inverted CD4/CD8 ratio also increased significantly above the age of 60. Interestingly, the proportion of individuals with an inverted CD4/CD8 ratio was found to be significantly higher in men, whereas the numbers of CD3+CD4+ helper and CD8+CD45RA+CCR7+ naive cells and the CD4/CD8 ratio were found to be significantly higher in women. These results highlight the importance of functioning of the thymus in the development of IRP and may partly account for the differences between sexes in terms of longevity.