Complex sentence: Many clinicians believe that andropause is not a valid concept, because men can continue to reproduce into old age, and do not usually show the same sudden and dramatic drops in hormone levels that are characteristic of menopause in women.
Split into simpler sentences: Many clinicians believe that andropause is not a valid concept, because men can continue to reproduce into old age. Their reproductive systems do not stop working completely in midlife, and therefore they do not show the same sudden and dramatic drops in hormone levels that are characteristic of menopause in women.