Final answer:
The sex ratio in a population is not fixed and can indeed vary among age classes, influenced by mortality rates, cultural practices, economic factors, and policies.
Step-by-step explanation:
The statement that the sex ratio in a population is usually fixed and does not vary among age classes is false. The sex ratio can indeed vary among different age groups within a population due to a variety of factors including gender-specific mortality rates, birth rates, cultural practices, economic factors, immigration patterns, and policies such as China's previous One-Child Policy.
For example, infant mortality rates can be higher for boys, leading to a higher ratio of girls in younger age classes. Conversely, because women tend to live longer than men on average, there may be a higher ratio of females in the older age classes. Additionally, migration for employment in certain industries can result in an excess male population in some regions, whereas the opposite might be true in other contexts.