Average age to start a PhD and correlation to future career
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: