What is an Age header?
How many seconds the response has been sitting in a proxy cache.
Age tells a client how long a cached response has been sitting in a proxy or CDN before it was served, which is genuinely useful for debugging why a website appears to be serving stale content even after you've pushed an update. A steadily climbing Age on content that should be updating regularly is often the first clue that a cache invalidation step in a deployment pipeline has quietly broken.
Common use case
An engineer debugging why a deployed fix isn't showing up checks the Age header on the response - a large, steadily growing number confirms the CDN is serving a stale cached copy rather than hitting the origin.
Example
Age: 3600History
Part of the original HTTP/1.1 specification, RFC 2068 (1997), designed to work alongside Cache-Control's max-age.
Did you know?
Its value is calculated cumulatively across every cache in the chain, so a response passing through several nested CDNs can report a much larger Age than any single cache actually held it for.