During the study, the scientists asked 72 men between 23 to 80 years to tap their index fingers as fast as they could for 10 seconds and measured the amount of myelin - a fatty sheath of insulation that coats nerve axons and allows for signalling bursts in our brains.
The loss of the myelin sheath insulating the nerves is the hallmark of some neurodegenerative autoimmune diseases.
The study showed that the tapping speed and the amount of myelin was at its peak at the age of 39 and started declining "with an accelerating trajectory" thereafter.
Lead researcher George Bartzokis, professor of psychiatry at UCLA, called the findings "pretty striking".
"That may well be why, besides achy joints and arthritis, even the fittest athletes retire and all older people move slower than they did when they were younger," Live Science quoted him, as saying.
Researchers have long been studying that brain aging might be primarily related to the myelin breakdown.
"Since in healthy individuals brain myelin breakdown begins to occur in middle age, there is a decades-long period during which therapeutic interventions could alter the course of brain aging and possibly delay age-driven degenerative brain disorders such as Alzheimer's," Bartzokis said.
They hope that further studies about this decline in fine-motor-skills speed might open new avenues for development of new treatments for aging brain.
The findings are detailed in the online version of the journal Neurobiology of Aging.