Being a bit older than Scott Adams, and older even than both Trump and Clinton, I have, perhaps, some qualification to comment on this theory, which in my view is mistaken. But before considering the question on the basis of any theoretical consideration or my personal experience of age, what of the historical evidence?
Consider Prince Mikhail Kutuzov, appointed Commander in Chief of the Russian Army as Napoleon embarked on the invasion of Russia in 1812. Aged 67, grossly overweight, within months of death from natural causes, Kutuzov ordered Russian forces to retreat in the face of the enemy, burning crops and grain stores as they went. Suffering from narcolepsy, Kutuzov had to be tied to his horse to prevent him falling off when he drowsed. Periodically, his staff officers would wake him and ask "what now," to which he would reply "continue the retreat."
Eventually, to satisfy the Tsar and his own staff, Kutuzov ordered the army to take a stand at Borodino. In the ensuing battle he lost a third of the Russian army, destroying a similar fraction of the French army in the furious one-day conflict. But before the following day dawned, Kutuzov had ordered a resumption of the retreat, his army falling back on Moscow, a city chiefly of wooden buildings which it burned, thus destroying all supplies of food and fuel. Kutuzov then ordered the army to retreat further to the East.
Carl, Gustaf, Emil Mannerheim, who in 1939, at the
age 72, commanded Finnish forces in the successful
resistance to the Soviet invasion of Finland in which
the Soviets lost a million men, to Finland's 25,000.
Or consider Winston Churchill. If you think Britain did well to stand against Hitler, then Churchill, who became Prime Minister only 21 days short of his 67th birthday, was surely the man to lead the nation. No other British politician could have so rallied the nation, or could have worked more effectively to bring America into the War at Britain's side.
Why in these and countless other cases have old leaders proved successful?
At a youthful 44, the age at which John F. Kennedy became President of the United States and Tony Blair became Prime Minister of Britain, a thoughtful person interested in philosophy, politics and economics might have read a couple of thousand books relevant to a life in politics or the military. At 64, the same person would have read thousands of books more. In addition, they would have worked for, with, or in supervision of many more people, and successfully or otherwise, they would have made many more practical decisions.
What this means is that, in early middle age, a leader will usually be pretty much a novice: still figuring out both things and people. But past 65, it is time to be in earnest. Then one is confident of what one knows and has judged of most challenges that leadership is likely to present. It is a time to ignore the briefing books and to give to the bright and energetic young people at one's command the direction, while leaving it to them to burn the midnight oil working out the details and organizing the action. then one should spend the evenings, Reagan-style, socializing with friends, watching old movies or going early to bed.
Consistent with this view of age and leadership are Winston Churchill's repeated youthful blunders. During World War One, aged 40, as First Lord of the British Admiralty, Churchill was responsible for the disastrous invasion of Turkey at Gallipoli, a blunder that cost him his cabinet seat. Ten years later, as Chancellor of the Exchequer, Churchill blundered again, fixing the pound sterling at a depression-inducing rate against gold.
True, there have been exceptional leaders of great youth: the empire builder, Alexander the Great, king of Macedon at 20, dead at 27; Henry V, victorious against France at Agincourt and Crecy, King of England at 26, dead at 36; and William Pitt the Younger, British Prime Minister at 24, dead at 36. But these are exceptions that test the rule. All were the sons of powerful men and imbibed from their fathers the rules of leadership. Though attaining power in youth, they were able to apply the rules of experience rather than having to to figure it all out for themselves, the fate of so many blundering middle-aged politicians who have spent most of their adult lives, not in leadership roles or learning by close proximity to a great leader the rules of statecraft, but in the sordid business of climbing the greasy pole to achieve a position of leadership.
The above is not, of course, to endorse either Clinton or Trump. There are plenty of old people, but few great leaders, potential or otherwise.