Uh, then you don't have a Leghorn. There are a couple breeds specifically bred for meat production that do as you were told by the ag folks. THOSE birds generally die long before they are 18 months old, because they are bred to be processed at less than six months of age, having reached the size they need to be. They often die of heart attacks because they cannot carry their bulk (and legs give out, too).
The life span of a regular chicken, not one bred for rapid growth, can be as much as 10 years, although most don't make it that long due to mishap. There are members here who have 14 year old chickens.....
A lot of people think chickens only live 2 - 3 years, because that's they're best laying years. Chicken factories cull their stock around 2 years of age, because egg production falls off afterwards, but old hens can lay eggs as long as they live. Just not very often.