When does perimenopause generally begin?

Late 40s/ early 50s. The average age of menopause, defined as the very last period, is 51 though it can vary anywhere from 45 to 55. Perimenopause generally refers to the 2 to 3 years around the time of the last menstrual period when the periods may become irregular and hormone swings may be more dramatic. Sxs vary from none to extreme hot flashes, breast tenderness, pms, menstrual migraines and mood changes.
40's. Average age of menopause is 51 and the transition on average between about 42-44. But for some women it can come much earlier, even in 30's. So there is a lot of variability. And for some women it does not happen till after 50.