Yet the economy is not in immediate danger.
And the maturity of the business cycle cuts both ways.
It makes a nonsense of Mr Trump's claims to be the author of American economic success.
But the economy is also capable of some welcome surprises.
America is not the only economy doing well.
For about a year, a synchronised global expansion, taking in Europe, Asia and the Americas, has been under way.
GDP growth in the euro zone, a region until recently synonymous with economic misery, is around 2.5%, despite slower population growth than America's.
But America stands out because of where it is in the cycle.
If it continues in 2018, this expansion will become the country's second-longest ever.
True, there are perils.
As the business cycle matures, there is more chance that the economy will overheat, because of bottlenecks in the jobs market;
or that the central bank overtightens in order to prevent things from running too hot.
The longer the economy keeps growing, moreover, the more scope there is for financial imbalances, such as excess debt or frothy asset prices, to build up.
Some warning signals are flashing.
The gap between long-term and short-term interest rates has narrowed, as it tends to before recessions.
Yet the evidence for overheating is thin.
Inflation has trended lower this year.
Wage growth has picked up a little, thankfully, but shows few signs of accelerating.
Pay would have to increase by quite a lot more before rising inflation is a real worry.