Why this should be is still a matter of hypothesis, but a range of tentative explanations has been put forward. One points the finger at myelin, a white, fatty substance that coats axons[轴突], the tendrils[卷须] that carry signals from one neuron to another.
Steady reductions in myelin as people age may be slowing down these connections. Another possibility, says Timothy Salthouse, director of the Cognitive Ageing Laboratory at the University of Virginia, is depletion of a chemical called dopamine['dəʊpəmiːn][多巴胺], receptor sites[受体部位] for which decline in number with advancing age.
on the wane:becoming less strong, powerful, popular, etc. （权力、人气等）衰落，减弱
There are signs that support for the group is on the wane.
tentative:(of a plan or idea) not certain or agreed, or (of a suggestion or action) said or done in a careful but uncertain way because you do not know if you are right （计划、想法、建议、行动等）试验（性）的，试探（性）的，暂时的
I have tentative plans to take a trip to Seattle in July.
a substance containing a lot of fat that forms a covering around nerves, especially those in the brain, protecting them and helping them to send signals effectively 髓磷脂（含大量脂肪，特别是在脑神经周围形成保护层，保障脑神经有效发出信号）
The myelin sheath around the neuron is damaged.
depletion:a reduction in something 减少；减小；缩小
the depletion of the ozone layer
Fortunately, there is some good news to go with the bad. Psychologists distinguish between "fluid intelligence", which is the ability to solve new problems, and "crystallised intelligence", which roughly equates to[同等看待;使相等] an individual's stock of accumulated knowledge.
These reserves of knowledge continue to increase with age: people's performance on vocabulary and general-knowledge tests keeps improving into their 70s. And experience can often compensate for cognitive decline. In an old but instructive study of typists[打字员] ranging in age from 19 to 72, older workers typed just as fast as younger ones, even though their tapping speed was slower. They achieved this by looking further ahead in the text, which allowed them to keep going more smoothly.
What does all this mean for a lifetime of continuous learning[终身学习,持续学习]? It is encouraging so long as people are learning new tricks in familiar fields. "If learning can be assimilated into an existing knowledge base, advantage tilts to the old," says Mr Salthouse. But moving older workers into an entirely new area of knowledge is less likely to go well.
assimilate:to become part of a group, country, society, etc., or to make someone or something become part of a group, country, society, etc. 吸收；融入；加入；（使）同化
The European Union should remain flexible enough to assimilate more countries quickly.
tilt:to (cause to) move into a sloping position （使）倾斜
He tilted his chair backwards and put his feet up on his desk.