This is something else you hear an awful lot about in regard to macarons. What we call “aged eggs” French pastry chefs simply call “eggs”, since they tend not to refrigerate theirs. They just get them very fresh, use them relatively quickly, and order more. Here we’re a little more uptight about maintaining egg freshness, which I don’t think is all bad. However it does put us at something of a disadvantage when it comes to whipping up egg foams.
Why? Because as eggs age, their whites get runnier. This doesn’t effect they way they taste or cook up, but it does affect the way they whip. Thin liquids can simply be agitated more briskly than thick ones. A whip will cut through a bowl full of water with much more force than it will through a bowl full of honey, if you follow me. That extra force, when applied to egg proteins, means a higher froth.
Being a skeptic by nature, I’m not totally convinced that aged eggs make that big a difference in a macaron batter. After all, part of making a macaron batter is popping a good deal of those bubbles. However aged egg advocates may have a point in that foams made from old eggs probably have a higher proportion of small bubbles in them, and those may make a contribution to the macaron’s subtle rise.
Age your egg whites by putting them in a bowl, covering the bowl with plastic wrap, and leaving the bowl out on the counter for about 24 hours. At room temperature, eggs age one day per hour compared to how they’d age in a refrigerator. By morning those whites will be good and runny, but will not have spoiled. Oh, and don’t fall for the myth that you can achieve the same effect by microwaving your whites for ten seconds or so. That may warm the whites, but won’t have any effect at all on their viscosity.