What's the difference between ageing and dementia?

As we age, our cognitive abilities decline gradually, but dementias are a fork off that path. Determining whether mild symptoms are due to normal ageing, mild cognitive impairment (MCI) or a progressive disease is difficult; a person may have the underlying biology of dementia, but not exhibit obvious symptoms. Cognitive tests and scans can be normal in early stages. It's thought that the disease process may occur for up to 20 years before signs appear.

Normal cognitive decline in ageing

As we age, our cognition – mental processes – naturally declines. The most notable change may be cognitive slowing, with one study showing that a 20-year-old is 75% faster than a 75-year-old at substituting symbols for numbers. With regards to language, vocabulary and verbal skills remain largely unchanged, but the ability to find a particular word can decline. Memories formed many years ago remain quite stable, but forming new memories can be difficult, and working memory (e.g. recalling a phone number to dial it) is particularly affected. The ability to focus on a single task is largely unchanged but there is increased difficulty with multitasking or switching attention.

These changes reveal a pattern: although our general knowledge and ‘crystallised intelligence’ (the ability to use skills and knowledge) are mostly unaffected, our fluid intelligence (the ability to think on the fly and solve new problems) suffers. 

Mild Cognitive Impairment

MCI exists in a transitional or grey zone between normal cognitive decline and dementia. The symptoms are similar to those of dementia – forgetfulness, impulsivity, irritability and some difficulty maintaining a train of thought – except that they are not serious enough to interfere with a person’s independent function or overall cognitive state, as a person is still able to do complex tasks. MCI is a risk factor for developing dementia and a proportion of people with MCI go on to develop dementia. 

Dementia: when cognitive decline affects daily functioning

In dementia, the features of cognitive decline become amplified. Rather than simply forgetting that a meeting was on, someone with dementia might not remember the meeting was ever planned. Personality changes, confusion, problems navigating familiar environments, and difficulty performing everyday tasks are common challenges. Inside the brain, the accumulation of deposits, such as amyloid-ß and tau in the case of Alzheimer’s disease (see page 6), cause damage to neurons. While these deposits may signal a neurodegenerative disorder, they can sometimes be found in cognitively normal people.