I have a ton of thread that was given to me. All colors and types but I do not know if it is safe to use it, at a guess I would say it must be at least five years old.

Yank on it. If it breaks, it's too old.

That's not old. The thread in my vintage quilt is 50 years old and doing fine. Tug on the thread before you use it. If it feels rotted, then pitch it, but I don't see how it could be in five years. 100% cotton will break faster than a poly or blend, but it won't feel rotted.

age has nothing to do with it- if it is strong- not discolored- still usable it's not too old-
if it breaks easily when held between your two hands and tugged it is not much good any more-
usually because of the way it was stored- not by it's age-
i've had 30 year old thread that is still fine- i've had thread 6 months old that was worthless-

There was a short video posted here that showed how to test older thread for strength/weakness. Maybe someone remembers the link? :D:D:D

I've seen new cheap thread that breaks easily. boo hiss
If yours is quality it should be fine.

If it breaks when you pull on it, you might just need to cut off the outer layer of thread. Once you get to the thread that has been more protected, underneath, it might be fine.

I regularly use thread from mom's stash with no problems.
Remember when lage spool of coats and clark thread was 79 cents that tells the age of some of this thread.

i got rif of all my old polyester thread only buy cotton now our church was sending sewing supplies to haiti so i gave it to them

I was given some a while back and every one was bad, of course I didnt realize it till after I started having machine issues. After yanking on every spool, I decided, I no longer except thread donations.