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.