Hey, if they are in your attic, all bets are off. My brother has a sleeping bag, a Swallow, that I bought in 1987, and it is in near-mint condition.
Feathered Friends Swallow Sleeping Bag
Swallow Sleeping BagHere is the general deal: For a down bag, theres no set lifespan. Buy a good-quality down bag like the Swallow (currently $344 in standard shell), keep it reasonably clean, leave it to expand when stored, and you can expect, well, lifetime service. As for my rules, give it a wash with down cleaner once every year or two, wear long underwear to bed so you dont grease up the lining, and store in a loose cotton bag. That will do it.
Synthetic bags have a bit less of a lifespan. Same rules apply: Clean it only when needed, do what you can so it doesnt need to be cleaned often (laundering will in time break down the fiber connections), and store it loose. But in time you will see the loft go down.
So, heres my advice for the bags you found. Give the bags a good shake and lay them on the floor. If they still are nice and fluffy (Unscientific Wild Ass Guess: five inches of loft for a 30-degree bag), then you are good. They arent that old and probably have roots in the modern era. If they are flat as pancakes, well, time to retire. Go buy a Swallow.
The is now online so you can get prepped for gift-giving seasoneven if everything you pick is for yourself!