How old must you be to be 'ancient'?

If you're a document that someone is trying to exclude under the federal hearsay rule, the answer is 20 years:

[Rule 803:] The following are not excluded by the rule against hearsay, regardless of whether the declarant is available as a witness: …

(16) A statement in a document that is at least 20 years old and whose authenticity is established.

But wait: That's likely to change, to a firm cutoff of 1997 or earlier, though I expect that this cutoff will change with future rules changes.