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."

美國的研究指出：我們的腦力在22歲進入巔峰狀態之后，在27歲開始衰退，這也標志著我們開始老了。

弗吉尼亞大學的Timothy Salthouse教授發現，我們的推理能力、空間想象能力以及思維的速度都在快三十歲時開始衰退。他同時指出，那些想去減緩或者逆轉衰老的治療可能需要很早就開始了。

他花了七年的時間研究2000名年齡在18-60歲之間的健康成人，這項研究最近發表在《老化神經生物學》雜志上。

為了測試大腦的敏捷度，實驗的參加者需要解決謎題、重復詞匯和故事的細節、同時指出詞語和符號的規律。

同樣樣的測驗也被醫生用于診斷老年癡呆癥的跡象。

12項測驗中的9項，出現最好成績的平均年齡都是22歲。而最早出現衰退的年齡是27，這時候反應速度、邏輯推理和解決圖像謎題的能力開始下降。

記憶這類能力，平均會一直保持較高水平直到37歲，而另一些需要知識積累的能力，比如詞匯測試或者常識測試，在60歲前會一直增長。

Salthouse教授說，他的發現指出“在健康、受過教育的成年人中，和年齡相關的某些認知能力在二三十歲時開始衰退。”

阿爾茨海默氏癥研究基金會的Rebecca Wood同意上述的觀點，她說：“這項研究指出有些思維能力的自然衰退開始得比我們預期的要早得多，它在二三十歲就開始了。”

“更多地了解健康的大腦如何衰退可以幫助我們了解像阿爾茨海默氏癥這樣嚴重的疾病中，大腦究竟除了什么問題。”

“阿爾茨海默氏癥并不是自然衰老的一部分，而是一種殺死腦細胞的身體疾病，它同樣影響著成千上萬65歲以下的人。”

“目前老年癡呆癥還是一種無法之余的疾病，為了給70萬患有老年癡呆癥的英國人希望，我們急切地需要進行更多的研究。”