Adds info about Age response header (#21132)

Author: angelampcosta

src/content/docs/cache/concepts/cache-responses.mdx

@@ -9,6 +9,12 @@ head:
 
 The `CF-Cache-Status` header output indicates whether a resource is cached or not. To investigate cache responses returned by this header, use services like [Redbot](https://redbot.org/), [webpagetest.org](http://www.webpagetest.org/), or a visual tool like [Chrome's Dr. Flare plugin](https://community.cloudflare.com/t/community-tip-dr-flare-debug-tool-for-cloudflare-chrome-extension/110166).
 
+:::note[`Age` response header]
+The `Age` response header is a header returned from cache that specifies the time in seconds that an asset has been in Cloudflare's cache. This value resets if the asset is revalidated, purged, or evicted and then re-cached.
+
+The `Age` header is only present for responses served from the cache. It will not appear on a cache MISS, dynamic traffic, or any responses that did not originate from the cache (for example, responses generated by a Worker that bypassed the cache).
+:::
+
 Below you can find a comprehensive breakdown of Cloudflare's cache response statuses.
 
 ## HIT