We all know they are bad. But consider the following: Person A: from age 10 onwards, this person consumes X amount of transitive fat per day. Person B: from age 10 to 35, this person has a wholesome diet with no transitive fat, then from age 35 onwards he consumes X amount of transitive fat per day. Who has the highest life expectancy? Person A eats transitive fats for a longer period. But he also ate them as a kid and perhaps his body got used to dealing with them. Person B eats less transitive fats in his lifetime, but starts later on at age 35 when his body has already adapted to a more healthy lifestyle. If a person quits smoking at age 65, does it increase or decrease their life expectancy?