Which then raises the question: if a teenager has managed the extraordinary feat of becoming a viable candidate, then they're clearly an outlier, and whatever you think about teenagers in general may not apply to them. Alexander the Great was... 20 years old when he became king and 22 when he began his invasions.
I searched for "teenage kings" and found https://www.history.com/news/6-child-monarchs-who-changed-hi... . Ptolemy XIII, from age "11 or 12": seems to have screwed things up. Fulin / "Shunzhi Emperor", taking power at 12, seems to have done a decent job. Elagabalus, becoming Roman emperor in A.D. 218 at 15, screwed up horribly. Pharaoh Tutankhamen, starting at "9 or 10", seems to have done well. Mary, Queen of Scots... I don't see much about decisions she made during her reign; she seems to have been trampled by more powerful enemies without it being obviously her fault. Baldwin IV of Jerusalem, from age 15: seems he did well. Incidentally, all of the above died in their 20s or younger (except Mary, who was locked up for life in her 20s).
So that's two screw-ups, three did-well, and one don't-know. The article says "who changed history", which means something significant happened, which might give us a biased selection—in many professions, the easiest way to do something very significant is to mess up something critical. I wonder if someone has made a more extensive and non-selected list.
Regarding this particular set, I also might say we should compare Elagabalus with other Roman emperors in the 200s A.D.—I'd guess there are other stinkers on that list... Looking at the others in the Severan dynasty... Elagabalus was succeeded by Severus Alexander (age 14!), who apparently did whatever his mother recommended, and seems to have done ok-ish (though the army grew to hate him and eventually overthrew him). Before Elagabalus was Caracalla, who co-ruled with his father Septimus (who did very well) starting at age 10 (!) and became emperor at 23 (technically co-emperor with his brother, but he had his brother killed that year), and was awful.
Hmm, interesting. Unfortunately the Severans after Septimus were all young, which makes it hard to tell between "the Severans being too young made them suck" or "the Severans after Septimus sucked". Let's look at another Roman dynasty... the ones after the Severan seem too short and interrupted by weirdness to judge... Let's try Flavian. Vespasian began the line, was a military officer before he was emperor, and did well; Titus, his son (age 40, also already a military commander), did well (until he died of a fever); Domitian, Titus's brother (age 30), seems to have been effective though possibly tyrannical.
It seems a main lesson from the first bunch of Roman emperors was, if the emperor chooses a successor for reasons other than "family", things tend to go well; if there is automatic inheritance, it tends to go badly. But it also seems to go along somewhat with age: Caligula became emperor at 25, Nero at 17.
It's possible that there's unexpected wisdom in the age-35 rule. Still... If someone had been tutored from a young age by great teachers (Alexander the Great was apparently tutored until 16 by Aristotle), then managed to gain a few years' experience running a large chunk of a mid-size company (perhaps by a parent bringing them along to meetings, having staff explain things, then having the kid do research and come up with his own proposals that he delivers in meetings, and ultimately grow into some CxO role) and, by all accounts, running it extremely well; and finally managed to rise to national prominence by, I don't know, identifying a national crisis and getting several companies to work together to fix it, and everyone who's interacted with him says he's great; and if he did this all by the age of 19, would you think he's a worse candidate than the last several presidents of the U.S.?