Of the yarn I procured firsthand, my oldest skeins are acrylic, and about 15-18 years old. Tweedy acrylic in cream, black, and red. Who knows what I was thinking of when I bought them... but there's a ton of it and I'm still trying to use it up/give it away.
Of the second-hand procurements... while there are no labels, I'm fairly sure some of it is older than I am... so over 30.
The second hand stuff ranges the field from acrylic to wool blend to cotton. Some of it is quite fine, but there's not enough of one type, let alone color, to make any useful lace article from it. I may just have to resort to crocheting some doilies with them.