diff --git a/src/backend/utils/adt/xid.c b/src/backend/utils/adt/xid.c
index 8ac1679c38..d2c637195c 100644
--- a/src/backend/utils/adt/xid.c
+++ b/src/backend/utils/adt/xid.c
@@ -98,6 +98,7 @@ xidneq(PG_FUNCTION_ARGS)
 
 /*
  *		xid_age			- compute age of an XID (relative to latest stable xid)
+ * This supports only 4-byte XIDs and is undocumented.
  */
 Datum
 xid_age(PG_FUNCTION_ARGS)
@@ -114,6 +115,7 @@ xid_age(PG_FUNCTION_ARGS)
 
 /*
  *		mxid_age			- compute age of a multi XID (relative to latest stable mxid)
+ * This supports only 4-byte XIDs and is undocumented.
  */
 Datum
 mxid_age(PG_FUNCTION_ARGS)