Mental powers start to dwindle at 27 after peaking at 22, marking the start of old age, US research suggests.

Professor Timothy Salthouse of the University of Virginia found reasoning, spatial visualisation and speed of thought all decline in our late 20s.

Therapies designed to stall or reverse the ageing process may need to start much earlier, he said.

His seven-year study of 2,000 healthy people aged 18-60 is published in the journal Neurobiology of Aging.

To test mental agility, the study participants had to solve puzzles, recall words and story details and spot patterns in letters and symbols.

The same tests are already used by doctors to spot signs of dementia.

In nine out of 12 tests the average age at which the top performance was achieved was 22.

The first age at which there was any marked decline was at 27 in tests of brain speed, reasoning and visual puzzle-solving ability.

Things like memory stayed intact until the age of 37, on average, while abilities based on accumulated knowledge, such as performance on tests of vocabulary or general information, increased until the age of 60.

Professor Salthouse said his findings suggested "some aspects of age-related cognitive decline begin in healthy, educated adults when they are in their 20s and 30s."

Rebecca Wood of the Alzheimer's Research Trust agreed, saying: "This research suggests that the natural decline of some of our mental abilities as we age starts much earlier than some of us might expect - in our 20s and 30s.

"Understanding more about how healthy brains decline could help us understand what goes wrong in serious diseases like Alzheimer's.

"Alzheimer's is not a natural part of getting old; it is a physical disease that kills brain cells, affecting tens of thousands of under 65s too.

"Much more research is urgently needed if we are to offer hope to the 700,000 people in the UK who live with dementia, a currently incurable condition."

美国的研究指出：我们的脑力在22岁进入巅峰状态之后，在27岁开始衰退，这也标志着我们开始老了。

弗吉尼亚大学的Timothy Salthouse教授发现，我们的推理能力、空间想象能力以及思维的速度都在快三十岁时开始衰退。他同时指出，那些想去减缓或者逆转衰老的治疗可能需要很早就开始了。

他花了七年的时间研究2000名年龄在18-60岁之间的健康成人，这项研究最近发表在《老化神经生物学》杂志上。

为了测试大脑的敏捷度，实验的参加者需要解决谜题、重复词汇和故事的细节、同时指出词语和符号的规律。

同样样的测验也被医生用于诊断老年痴呆症的迹象。

12项测验中的9项，出现最好成绩的平均年龄都是22岁。而最早出现衰退的年龄是27，这时候反应速度、逻辑推理和解决图像谜题的能力开始下降。

记忆这类能力，平均会一直保持较高水平直到37岁，而另一些需要知识积累的能力，比如词汇测试或者常识测试，在60岁前会一直增长。

Salthouse教授说，他的发现指出“在健康、受过教育的成年人中，和年龄相关的某些认知能力在二三十岁时开始衰退。”

阿尔茨海默氏症研究基金会的Rebecca Wood同意上述的观点，她说：“这项研究指出有些思维能力的自然衰退开始得比我们预期的要早得多，它在二三十岁就开始了。”

“更多地了解健康的大脑如何衰退可以帮助我们了解像阿尔茨海默氏症这样严重的疾病中，大脑究竟除了什么问题。”

“阿尔茨海默氏症并不是自然衰老的一部分，而是一种杀死脑细胞的身体疾病，它同样影响着成千上万65岁以下的人。”

“目前老年痴呆症还是一种无法之余的疾病，为了给70万患有老年痴呆症的英国人希望，我们急切地需要进行更多的研究。”