Average age to start a PhD and correlation to future career Which is the average age in which persons start their PhD? Is this age in some way related to the post PhD plans a person does?

In my (European) experience I have 3 groups of people, with the relative distributions:


(70% of the total) Those who started right after graduation (let's say when they were 24) without any job experience. Most of them (with exceptions, of course) are a bit fed up with research and willing to try a "real world job experience"
(25% of the total) Those who started 2,3,4 years later (so between 24 and 29), taking a full time phd. Generally people belonging to this group seem to be more motivated for academic career, even if starting a phd when one is 29 years old might be a bit late (or not?)
(5%) Those who start in their 40ties with the idea of taking a part time PhD as an out of job activity, and don't plan to leave their job.


Clearly there are all the possible shades between the above categories.
There are many confounding variables (field of study, country, citizenship status, sex, race/ethnicity, probably many others) that correlate with both age at doctorate (as seen below) and post-PhD plans. This makes it difficult to directly measure the relationship between age and post-PhD plans.

The median age at doctorate varies by country and by field. Here's some OECD data:



It also varies over time. In the US, from 1978-2003, in the science and engineering fields:



And in the non-science and engineering fields:



See this report for more details.

For the US, we have data on the distribution of students' age, not just the median, and it's also broken down by  sex, citizenship status, and ethnicity and race.  From 2014: