Perimenopause can begin anywhere from the late 30s, and into the early 50s. There is no exact time that women begin to go into menopause. What I always tell women is that it is important to begin preparing for menopause in the late 30s and throughout the 40s. A woman recently said to me when I said this that she doubted women would listen in the 30s and 40s when they are feeling great. My answer to her was that I absolutely listened and began to strengthen my endocrine, immune, circulatory and musculoskeletal systems at the beginning of my 40s. I had worked with women for so many years as a herbalist and bodyworker observing symptoms of change and listening to their complaints and frustrations and helping them through their menopause years. I saw what worked for some of them and not for others. I could feel their bone quality change. I could feel their joints tighten. I listened very carefully to how menopause was affecting their lives. With all of this experience with women, I simply embarked on building my body to it would ease through menopause. I went through a relatively pain-free and easy menopause without hot flashes, a 10-pound weight gain, joint problems or depression. I definitely noticed my memory lapse, which is a classic part of menopause and I still work on that today during post menopause.
So... I highly recommend that women start young to be able to breeze through the menopausal years. What worked for you when you were younger simply stops working. The entire being is asking for you to change the way you do things.