Well chickens like many other species are imprinted at birth /hatching/creation with a certain amount of eggs. And basically when they have layed those eggs thats it for laying. That's why artificially kept (Battery type) birds are terminated at 18 months as they have been used as egg laying machines, not only have they used up most of their imprinted eggs. the quality of the shell is often reduced.
I have a blue Cochin who is 10 years old and still lays. I very rarely keep birds longer than 3 years (ex-batts excluded) but this one has one wattle bigger than the other, but produces fantastic chicks, so it's worth keeping.
So I would say the upshot is the more they lay in a season the sooner they will use up their allotted/imprinted eggs. So how long is a piece of string really