Posted on 10/07/2010 | Comments (). Print. The average American woman enters menopause by age 51, but the hormone level shifts that trigger menopause can actually begin years, even decades earlier. Known as perimenopause, this condition can begin as early as age 35, and lasts anywhere from a few months to 10 years.Perimenopause vs. Menopause.osted on 10/07/2010 | Comments (). Print. The average American woman enters menopause by age 51, but the hormone level shifts that trigger menopause can actually begin years, even decades earlier. Known as perimenopause, this condition can begin as early as age 35, and lasts anywhere from a few months to 10 years.