The fact that older eggs float is because the eggs lose moisture in time, which is replaced with air. I have some plastic, self-sealing egg "cartons" that prevent the drying out, and it takes them a lot longer to start floating (I consume at least 2 eggs a day every day, as a t2 diabetic, it is important to start each day w/protein)...so I have a good turnover, mostly store-bought supplemented with excess from my friend, when I'm lucky. The FDA has, on it's site, the guidelines: how old they can be and still be AA, then they become grade A, how old Grade B can be (I won't eat them on principle). Also depends on storage temp, also on that site. Months and months. Look it up and let me know how scared it makes you.