I think the more appropriate thing to evaluate here is the 6 people who used it previously. 1 person can thrash a rope
Provided it was used infrequently, stored well, and cared for, and undergoes an inspection. I'd probably use it depending on it's diameter. A 25 year old 11 or 12 mil rope that's been treated kindly is still probably safe for normal use. My 120' 11mm was a hand me down rope. I've got no idea how old it is, but it's at the very least 12 years old, and realistically probably more like 20. I use it without worry. It's still in good shape and been stored well. I have a feeling that if I cut off a section and took it to be tested it would still test to appropriate limits. Have I done so? No. That being said, it was only ever used by one person previously and I know exactly where and how it was stored.
I've clipped into all kinds of fixed lines and rap anchors over the years. And climbing fixed lines (if I don't know who put them there) always gives me pause. So what to do? Subject it to the Fat Boy Test, cross your fingers, and cave on. I use the FBT at work a lot, substituting a z drag, or more bodies when I don't actually have a Fat Boy handy. Will it be strong enough? Probably. You just have to decide if "probably" is an acceptable answer
to you.