I can't say I agree. Actually we don't know all that well how long people lived during paleolithic times, when hunting and gathering was done and more meat eating common. There are some skeletons found from that time, but not many. What is known is that meat eaters tended to be taller, have stronger bones and formed teeth with few cavities. It is believed that they were healthier than their farming relatives. We do know that Egyptians, Romans and those in the middle ages, which ate somewhat similar to us lived into their 20s and early 30s.
I guess the item researchers point out is that human females developed menopause, which is a rare trait to have in the animal kingdom. The majority of females are able to reproduce right up till death. At some point during hunter gather times human females were living long enough that menopause was an advantage to have.
The theory that I read on this from Professor Jared Diamond and his series of books has to due to raising humans being uniquely tiring. It became advantages to be and to have grandparents to help in the raising of children.
As for meat eating today verses vegetarians, there doesn't seem to be much difference in longevity in todays world.
"Mortality in British vegetarians: results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford)1,2,3,"
http://ajcn.nutrition.org/content/early/2009/03/18/ajcn.2009.26736L.short
And not to over do it! But I remember also about the diet of the Irish and this mention from The Oxford Companion to Irish History - from the diet section I thought they had an interesting mention, page 147
"...Dietary changes were not always accompanied by improvements in nutritional standards. Paradoxically, at the lower end of the income scale the transition from a monotonous menu to more varied fare resulted in a fall in nutritional quality. The pre-famine potatoes and milk regime had been rich in almost all nutrients, but when it was replaced by cereals, bread, butter and tea, the nourishment was inferior. The new pattern was particuarly prevalent in towns where death rates from diseases of poverty were high. Contemporary commentators noted the deterioration in the nation's health and many ascribed it to poor diet..."