i really don't think 'age' has anything to do with it- it is more the thread itself- and the way it's been stored.
i've been using thread my mom bought from a going out of business store in San Diego in 1991- 20 years ago...and who knows how long it had been in the store.
it is still a good strong thread- although the supply is dwindling.
i have purchased brand new thread that was terrible- broke ever couple inches and was useless- so---it's not the age that really matters.