You don't see many really old, obese people whereas you see a lot of old thin people. It is reasonable to assume, exceptions aside, that obesity kills.
Unless you reach a certain age, it has been said. When it comes to seniors, research has reported an "obesity paradox" concluding that, at age 65 and older, having an elevated BMI won't shorten your lifespan, and may even extend it. A new study took another look at the numbers, finding the earlier research flawed. The paradox was a mirage: As obese Americans grow older, in fact, their risk of death climbs.