Why must care be taken in separating the egg yolks from the egg whites and those only very fresh eggs are used in sponge and chiffon cakes

Why don't mix yolks and whites? Because if the yolks get into the whites, their fat will inhibit the whites whipping up.
Why use fresh eggs? I think because these whip up better than older ones, but haven't found confirmation in searching around. Maybe others know more.