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THE youngest baby boomers are already past 40, and have probably started to notice changes in their mental abilities. Many of the older ones will be well on their way to full-blown dementia. So now is probably a good time for one among them to point to the bright side of growing old.

Elkhonon Goldberg is a clinical professor of neurology at New York University School of Medicine. His theory is that while some mental skills deteriorate as you age 鈥� notably the ability to recall recent events 鈥� others improve. Foremost among these, he argues, is pattern recognition, an important component of wisdom.

Pattern recognition is useful for solving problems. It is the ability to see that the object or problem confronting you belongs to a group of similar objects or problems, and is therefore likely to have similar properties. So you can solve the problem on the basis of prior experience, without knowing very much about it.

It鈥檚 a complex skill 鈥� so complex, in fact, that researchers in artificial intelligence have so far failed to incorporate it into their robots. Rodney Brooks, head of the computer science and artificial intelligence laboratory at the Massachusetts Institute of Technology, said earlier this year that no robot really understands the objects it encounters outside the lab because, in effect, it cannot categorise.

A toddler, by contrast, can recognise a cup as a cup, even if its shape or size changes, and knows what to do with it. As the child grows up, their notion of a cup becomes more sophisticated. Pattern recognition appears to be partly hard-wired, partly learned. The longer you live, the more examples you can add to each category of object or problem, and the better your chances of solving a problem. Unlike other skills, claims Goldberg, it doesn鈥檛 fall off when your grey matter shrinks below a certain critical mass.

The reason, he says, is that the kind of memories that underlie pattern recognition are generic memories: those that provide information about the common features of whole classes of things 鈥� the four-leggedness of dogs, to give a simple example. And these generic memories appear to be relatively resistant to ageing. So when it comes to problem-solving, older people have this huge reservoir of generic knowledge to guide them, and finding solutions becomes more intuitive as a result.

This is an optimistic book, and optimism is a great thing. There is even some evidence that a positive mental attitude can help ward off cognitive decline, in which case it may be worth reading the book for its cognitive-enhancement properties alone. Goldberg鈥檚 hypothesis is hardly fashionable: those who see ageing as good for the brain are about as numerous as those who think the hole in the ozone layer has nothing to do with humans. Still, older brains are the ones that society tends to hold in the highest regard, so it is interesting and worthwhile to ask what the ageing brain has that younger brains do not have, and why some brains age better than others.

As the population ages, the baby boomers are pushing the definition of middle age ahead of them. In terms of influence, if not strictly of cognitive acuity, the 40 to 70-year-olds of the developed world are at the peak of their powers. Look at Alan Greenspan, who has been chairman of the US Federal Reserve for 18 years, and will celebrate his 70th birthday next year. Or Nelson Mandela. Then again, look at Ronald Reagan and Winston Churchill, both of whom only relinquished the reins of power when their mental collapse was already painfully obvious to those who looked to them for leadership.

But there is another good reason for asking what happens to the brain as we get older. Even the ageing brain retains some plasticity, and there is growing evidence that not only regular physical exercise, but also regular mental exercise 鈥� doing tasks that demand sustained attention or planning, or just the flexibility to play a game in which the rules occasionally change 鈥� can ward off the cognitive decline associated with old age, and possibly even reverse it. 鈥淯se it or lose it鈥� may yet become the motto of the baby boomer generation.