docs: add source age policy

docs/SOURCE-AGE-POLICY.md

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+# Source Age Policy
+
+Open Problem Lab should treat source age as evidence metadata, not as a cosmetic citation detail.
+
+A source can be old and still valid. A source can be recent and still weak. The point of this policy is not recency worship. The point is to make time-sensitivity explicit so reviewers know when a claim depends on information that may have changed.
+
+## Claim classes
+
+Use the least strict class that honestly fits the claim.
+
+| Class | Meaning | Example refresh expectation |
+|---|---|---|
+| `stable` | Conceptual, historical, or physical background unlikely to change quickly. | Review when challenged or superseded. |
+| `slow-changing` | Structural facts, long-run datasets, institutional context. | Recheck when source is older than 24 months. |
+| `moderate-changing` | Program status, burden estimates, policy, funding, implementation landscape. | Recheck when source is older than 12 months. |
+| `fast-changing` | prices, active outbreaks, conflict, weather, current office holders, live service availability, recent regulations. | Recheck before use; stale after 30-90 days depending on domain. |
+| `snapshot` | A claim intentionally tied to a specific date or version. | Preserve date; do not silently generalize. |
+
+## Minimum citation note
+
+For time-sensitive claims, add a note in the evidence record, issue, PR, or review comment:
+
+```markdown
+**Source age class:** stable | slow-changing | moderate-changing | fast-changing | snapshot
+
+**Source date:** YYYY-MM-DD or source publication year.
+
+**Checked on:** YYYY-MM-DD.
+
+**Staleness risk:** Why this source might become wrong.
+
+**Refresh trigger:** What event, date, or reviewer concern should force a recheck.
+```
+
+## Reviewer guidance
+
+Reviewers should ask for source-age metadata when:
+
+- a claim uses numbers that could change;
+- the claim supports prioritization;
+- the claim describes policy, funding, implementation status, or institutional behavior;
+- a source is old but used as if current;
+- an agent uses a recent-looking upload or mirror without checking the original source date.
+
+Reviewers should not reject a source solely because it is old. They should reject stale use of an old source when the claim needs current evidence.
+
+## Agent guidance
+
+Agents should not infer freshness from retrieval order, upload time, or search ranking.
+
+Before treating a source as current, an agent should identify the source date and ask whether the claim class is stable, slow-changing, moderate-changing, fast-changing, or snapshot.
+
+If the source date is missing, say so.
+
+## Anti-patterns
+
+Avoid:
+
+- citing a current webpage for an old embedded report without naming the report date;
+- treating a dataset release date as the date of the underlying measurements;
+- using old policy documents as current policy;
+- using recent news as durable evidence;
+- hiding time boundaries in prose instead of metadata.
+
+## Acceptance test
+
+A reviewer should be able to answer:
+
+1. How old is the evidence?
+2. Does age matter for this claim?
+3. When should it be checked again?
+4. What would make it stale?
+
+If the artifact cannot answer those questions, it is not yet time-aware evidence.