Explanation:

The age structure of a population is primarily determined by birth rates, death rates, and migration. A high proportion of young population indicates a specific demographic profile. \nOption A: A 'high birth rate' means many new individuals are being born, which directly contributes to a larger proportion of young people in the population. When a population has a large share of young individuals, it is characterized as a 'youthful population'. This statement accurately describes the demographic situation. Therefore, this option is correct. \nOption B: A 'low birth rate' would lead to fewer young people entering the population, eventually resulting in an older population structure, not a youthful one. Thus, this option is incorrect. \nOption C: A 'high female population' doesn't directly imply a youthful population; it refers to the sex ratio. 'Ageing population' is the opposite of a youthful population, characterized by a higher proportion of older individuals, typically due to lower birth rates and increased life expectancy. Thus, this option is incorrect. \nOption D: A 'low birth rate' would indeed lead to a population that is 'not youthful' (i.e., aging or mature), but this contradicts the premise of having a 'high proportion of young population'. Thus, this option is incorrect.