The Age response header conveys the sender’s estimate of the time since the response was generated or successfully validated at the origin server.

Syntax

The value is a non-negative integer representing time in seconds:

Age: 3600

In the example above, the response was generated 3,600 seconds (1 hour) ago and served from an intermediary cache.

How proxies calculate Age

According to RFC 9111 Section 5.1:

  • When a response is generated directly by the origin server, the Age header is typically 0 or omitted.
  • When served from an intermediary cache (CDN, reverse proxy), the cache calculates the elapsed time since generation and sets Age.

Determining cache hits vs. misses

  • Age: 0 or missing: Origin response (cache miss or dynamic bypass).
  • Age: 120: CDN cache hit; content has been cached for 2 minutes.

How to inspect with curl

Observe Age and cache headers using curl:

curl -i https://api.example.test/v1/static/config.json

Key takeaway

The Age header indicates how many seconds a cached response has lived in intermediary caches. It is an essential diagnostic tool for verifying CDN cache hits and monitoring cache expiration relative to max-age.