When whisking eggs for your baking it is always best to use the freshest eggs possible.
Eggs, when fresh, are acidic and this acidity means that the egg proteins are tightly knit. You can see this egg is fresh because the yolk is plump and round and the inner white is clearly defined and is holding onto the yolk tightly – an indication that those protein bonds are keeping it from spreading on the plate.
As an egg ages, it becomes more alkaline and the proteins start to pull away from each other, causing the yolk to lose it's form and the white to become thinner and more likely to spread on a plate when cracked.
So why is this important when whisking eggs? Less fresh eggs will whisk to a foam more quickly than fresher ones, and you’ll achieve a slightly greater volume, but the resulting foam will be less stable than one made with fresh eggs. A fresh-egg foam will be more stable thanks to the strong protein bonds and therefore more likely to holds its shape and retain the air you have incorporated through whisking.