Usually in late forties and early fifties. Earlier if the ovaries were removed by surgery, like, if they had to remove the womb for some reason in a woman who has completed the family and is in her mid-late forties, some gynaecologists also remove the ovaries.
If it's earlier than that, the woman has to be investigated to exclude possible causes, because it can be due to illness in, say, ovaries, that produce female hormones. Since the female hormones have a protective effect against heart disease etc. she may also have to be put on hormonal supplements.
You are considered to be in Menopause when your period / menses has stopped fully for 24 months. When your period stops for 8 months and then restarts for the next 2 months but goes away again = that is not menopause. You have to be fully free from menses for 24 months to be considered in Menopause.