Complex sentence: The age of 40 was chosen as the cut-off point for a diagnosis of POF somewhat arbitrarily--as all women's ovaries decline in function over time, an age needed to be chosen to distinguish usual menopause from the abnormal state of premature menopause.
Split into simpler sentences: The age of 40 was chosen as the cut-off point for a diagnosis of POF. 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.