Complex sentence: This age was chosen somewhat arbitrarily, as all women's ovaries decline in function over time, however an age needed to be chosen to distinguish usual menopause from the abnormal state of premature menopause.
Split into simpler sentences: This age was chosen somewhat arbitrarily, as all women's ovaries decline in function over time. However an age needed to be chosen to distinguish usual menopause from the abnormal state of premature menopause.