Median age at doctorate by field of study and country?
In my opinion, there are two approaches to answering this question. The first (which seems that the OP expects here) represents references to publications, containing global summary statistics on doctoral graduates across the indicators of interest, such as age, field of study and country. In this category, IMHO the most well-known and comprehensive is the Education at a Glance report by OECD. AFAIK, the latest such report is Education at a Glance 2014: OECD Indicators, which, along with supplementary materials (i.e., highlights), can be accessed in various formats from this page.

The second, open data, approach, which while not requested, I recommend to those wanting to perform their own statistical data analysis, which might be very interesting especially by combining the indicators of interest with other socio-technical and economical indicators, widely available from open data repositories. In particular, for this specific question / category, I would suggest analyzing the following two open data sets from OECD, which can/should be combined in order to perform needed analysis: Graduates by field of education and Graduates by age.