Men who are less fit and/or have a lower IQ at the age of 18 may have an increased risk of developing early-onset dementia, a new study suggests.

When symptoms of dementia appear before the age of 65, this is referred to as early-onset dementia. An estimated 3,500 people in Ireland are affected by this condition - some are only in their 40s.

Swedish researchers analysed data from 1.1 million young men and found that those who had poorer cardiovascular fitness at the age of 18 were 2.5 times more likely to develop early-onset dementia later in life.

Those with a lower IQ at the age of 18 were four times more likely to develop the disease.

Meanwhile, those with both poorer fitness levels and a lower IQ at the age of 18 were seven times more likely to develop it.

"Previous studies have shown the correlation between cardiovascular fitness and the risk of dementia in old age. Now, for the first time, we can show that the increased risk also applies to early-onset dementia," commented head researcher, Jenny Nyberg of the Sahlgrenska Academy.

The results stood even when other factors were taken into account such as medical history and hereditary factors.

The researchers emphasised that because people who develop early-onset dementia are often of working age, with children still living at home, the consequences for them can be even more serious. Despite this, they are a relatively overlooked group.

"This makes it important to initiate more research into how physical and mental exercise can affect the prevalence of different types of dementia. Perhaps exercise can be used as both a prophylactic and a treatment for those in the risk zone for early-onset dementia," Ms Nyberg said.

Details of these findings are published in the journal, Brain.