Does vegetarianism affect life expectancy? Is an average vegetarian supposed to live longer just because of their diet?
Yes, according to a paper by the American Society for Clinical Nutrition. (NIH link here)
Results: Our review of the 6 studies found the following trends: 1) a very low meat intake was associated with a significant decrease in risk of death in 4 studies, a nonsignificant decrease in risk of death in the fifth study, and virtually no association in the sixth study; 2) 2 of the studies in which a low meat intake significantly decreased mortality risk also indicated that a longer duration (≥ 2 decades) of adherence to this diet contributed to a significant decrease in mortality risk and a significant 3.6-y (95% CI: 1.4, 5.8 y) increase in life expectancy; and 3) the protective effect of a very low meat intake seems to attenuate after the ninth decade. Some of the variation in the survival advantage in vegetarians may have been due to marked differences between studies in adjustment for confounders, the definition of vegetarian, measurement error, age distribution, the healthy volunteer effect, and intake of specific plant foods by the vegetarians.
TL;DR: 1. less meat = less risk of death, 2. less meat for a longer time = an average of a 3 year longer lifespan + less risk of death, 3. after 90 years of no meat it stops decreasing your risk of death.
Conclusion: Current prospective cohort data from adults in North America and Europe raise the possibility that a lifestyle pattern that includes a very low meat intake is associated with greater longevity.
So yes, scientific research says that vegetarianism could help you live longer, but it primarily decreases your risk of death. (i.e. you might not live to 105, but you probably won't die at 55)
I just read through everybody's comments on this highly significant life/death question. My observation is that the high number of influencing variables will skew the conclusion of any study. With only a small percentage of the total population being vegetarian, having no prevalent marker to identify who they are, a conclusion can't be drawn that group either. Populations that have a significant vegetarian count may also have other factors that skew the interpretation as to why they die when they do.
So far, now a vegetarian myself for over 35 years, my conclusion is that being a vegetarian will add to my life expectancy. That's just because I feel good about what I eat (most of the time). I'm not relying on any study for that feeling but do take into account the endless info-snippets that I am bombarded with from my family and friends. There I have drawn my own conclusion which is that no good can come from eating meat. Add, that the no-meat diet that just feels good and extends to my overall life style and healthy, or not, decision making.
So to answer the question, I'll draw an analogy to what my stock broker tells me about investing, "If it feels good then go with your gut". There's probably a lot of underlying reasons for those feeling and I believe your gut feel is major in answering this question too.