One important reason is the big delay - and advantage - women have over men in terms of cardiovascular disease, like heart attack and stroke. Women develop these problems usually in their 70s and 80s, about 10 years later than men, who develop them in their 50s and 60s. For a long time, doctors thought the difference was due to estrogen. But studies have shown that this may not be the case, and now we know that giving estrogen to women post-menopause can actually be bad for them.
One reason for that delay in onset of cardiovascular disease could be that women are relatively iron-deficient compared to men - especially younger women, those in their late teens and early 20s - because of menstruation. Iron plays a very important part in the reactions in our cells that produce damaging free radicals, which glom onto cell membranes and DNA, and may translate into aging the cell. In fact, in our diets, red meat is the main source of iron, and lack of iron is probably one major reason that being vegetarian is healthy for you. There was a very good study looking at the intake of red meat and heart disease in Leiden in the Netherlands: in regions where people didn't eat red meat, those populations had half the rate of heart attack and stroke compared to the populations that did eat red meat.