Why does the balance of males and females tend to change later in life?

The shift in the balance of males and females as a population ages is primarily driven by biological differences in mortality rates between the sexes.12

While slightly more males are born than females—typically at a ratio of about 105 or 106 males for every 100 females—males experience higher mortality rates at virtually all ages after birth. This biological disparity means that the initial surplus of males gradually decreases throughout childhood and adulthood.12

As a result of this higher male mortality, the sex ratio shifts over time:
* Childhood: Males outnumber females, though the gap narrows as age increases.1
* Adulthood: At some point in the adult life span, the numbers of males and females become equal.1
* Older Age: Beyond the age of approximately 50, the number of females becomes disproportionately large compared to males.12

While these trends are generally consistent, they can be influenced by environmental, social, and economic factors, such as unequal access to resources or health care, which may alter mortality rates in specific regions.13