Would we expect the executive format (presidential, parliamentary, etc.) to affect the age at which an executive leader assumes office?
I might expect either no effect (too many other variables might swamp the format) or a positive effect of PMs. We know that prime ministers tend to have more “insider” experience (e.g. as cabinet minister, legislator, etc.) than presidents have. It takes time to get experience, so prime ministers might tend to be older.
However, exploring the data a little bit, what we actually see is the reverse. PMs tend to be younger. The effect is stronger in “third wave” democracies than in a set of both older and newer democracies. ((The president-vs.-PM effect gets slightly stronger when executives of semi-presidential systems are included, but what is shown here are the data from pure-format systems.)) The effects are statistically significant, though somewhat less so when the older democracies are included.
Why? An “older statesman” effect, whereby voters are more likely to elect older candidates? A compensation effect by parties whereby, aware of an electoral advantage in nominating relatively less “insider” politicians, they select older candidates whose “type” might be better revealed compared to those who are both outsider and young?
That prime ministers tend to be younger in parliamentary systems of the third wave than in older democracies might mean that political careers overall start younger when the democracy is younger. We already found (Samuels and Shugart, 2013; see prior link) that there is no observable difference in length of prior legislative experience of third-wave parliamentary PMs compared to those in older parliamentary democracies. But there is a difference in average age of PMs across eras. ((If we compare only older democracies, PMs do tend to be older than presidents. But the effect is not close to significant.))
So, to summarize, it seems that parties in parliamentary democracies in the third wave are promoting politicians to the top job who are younger than their counterparts in older democracies (but not less experienced as legislators). In presidential systems, on the other hand, in both new and old democracies there is a tendency for successful presidential candidates to be somewhat, and significant statistically, older.
There is, by the way, no time trend. That is, neither type of executive tends to start office older (or younger) as the democratic regime itself becomes older.
Data summaries (average age at start of tenure); all tests exclude executives who started their tenure before the date the regime became democratic (though the effects work even if these few politicians are included).
Third-wave, pure-format, democracies
Pres: 56.3 (n=84)
PMs: 51.1 (n=109)
Third-wave and older pure-format democracies together
Pres: 56.0 (n=148)
PMs: 54.7 (n=399)
Parliamentary PMs in older and newer democracies
Not third wave: 55.97 (n=290)
Third wave: 51.1 (n=109)
Based on the Samuels and Shugart biographical data.