求翻译：从20世纪70年代末开始，由于实行了以提高人口质量、控制人口数量为目标的计划生育政策。中国得以在短时间内完成从高出生率、高死亡率、高自然增长率到低出生率、低死亡率、低自然增长率人口转变过程。以这种变化相随的人口年龄结构转变，老年人口迅速提高，到本世纪伊始，中国用不到30年的时间走完了发达国家经过上百年才完成的向现代人口增长模式转变的过程，与同等收入水平的发达国家相比，中国较早实现了人口转变过程，但是也过早迎来了人口老龄化，产生了其他国家未曾遇到过的问题，构成了特殊的政策挑战。是什么意思？

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From 20th century at the end of 70's starts, because implemented take improved the population quality, control population quantity as the goal birth control policy.China can complete in a short time from the high birth rate, the high mortality rate, the high natural rate of growth to the low birth r

From the beginning in the late 1970 of the 20th century, due to introduced to improve its quality, the goal of controlling population's family planning policy. China to finish in a short time from a high fertility and high mortality rate, high natural growth rate to a low birth rate, low death rate,