Can frozen egg whites be used in a macaron recipe? I put a few egg whites in the freezer a few couple months ago (frozen in an ice cube tray, then transferred to a freezer bag).  Would they be viable for using in a macaron recipe (either French or Italian)?  Or do I really need fresh ones that have been aged?

I've seen a few sources that claim it will probably work fine, while another source claims they will be too watery to work (for a French recipe, anyway).
Yes, you can, but the results may not be the best, when it comes to tenderness and aeration of the finished (baked) product. Coagulated egg products are mainly used for omelettes, scrambled eggs and souffles especially for mass production, or great quantities.

In cooking, but especially in confectionery making, fresh eggs are usually used, especially for preparations that are very delicate, like macarons.