I'd love to hear what sources these claims are from. I'm vegetarian, and every time I've come across these "vegetarians live longer" claims it's from dubious sources that cite random tiny studies in isolation.
I could believe modern factory farming, preservation, and mass-preparation techniques make meat much less healthy (than it used to be in our ancestral days), but a general "meat cooking brings carcinogens [to a level that matters, beyond plant cooking does]" sounds pretty unlikely too.
Need to control for healthy habits also associated with healthier diets, such as exercising, not smoking, etc. It should also eliminate any participants with clearly unhealthy habits like poor diets consisting of fast food and living extremely sedentary lifestyles (different from just not exercising).
There is no large long-term double-blind controlled study comparing a vegetarian/vegan diet to some omnivore diet - and there never will be.
The only diet studies are observational, and they are unreliable in all kinds of ways. The famous Blue Zones book has about the validity of Rich Dad Poor Dad or The Millionaire Next Door. But the book continues to sell, and at a macro level, its advice is not terrible, but it's also not "the only way."
Surprisingly (to me), there is some evidence that some people are genetically better disposed to eating a vegetarian diet than others.
