It seems that this is not formally supported:
https://trac.nginx.org/nginx/ticket/146
quote from linked ticket:
As far as I understand RFC 2616 a HTTP 1.1 proxy server must send an 'Age' header for responses generated from its own cache (see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.6). However nginx 1.1.18 seems not to send this header in case it sends a reply generated from its cache and 'proxy_http_version' is set to '1.1'.
Does anyone know of a work-around? this header seems to be required by the spec so I'm surprised that there's no way to add it.