AgeAge is a header stating, in seconds, how long a response has been sitting in a cache since it was generated at the origin. It is typically added when a shared cache like a CDN or proxy serves a stored response.
Its presence tells you the response came from a cache rather than being freshly produced by the origin.
A cache computes a response's remaining freshness as the freshness lifetime (e.g. Cache-Control: max-age) minus the current Age. So Age accumulates through cache layers, growing as it passes more proxies. When Age reaches max-age, the response becomes stale and subject to revalidation.
An Age near 0 means a fresh response just fetched from the origin; a large value means a long-cached one. This lets you diagnose cache hits and the behavior of your cache layers.
Age must be read together with Date and Cache-Control to be precise. Date is when the origin produced the response and Age is time spent in cache since, so combining them yields an accurate freshness judgment.
Age: <seconds>e.g. Age: 87