Stroke rates are falling, according to a major new study.
The incidence of strokes in a large area of south London fell by 39.5% between 1995 and 2010, from 2 47 per 100,000 population to 149.5.
Rates fell in men, women, white groups and those aged more than 45 - but not in those aged 15 to 44, or black groups.
Researchers from King's College, London investigated data in the South London Stroke Register, which covers an area with a population of more than 350,000.
Between January 1995 and December 2010, 4,245 patients with first-ever stroke were registered.
The average age of onset of stroke decreased from 71.7 years to 69.6.
There were significant increases in the proportion of 15-44-year-olds, from 5.1% in 1995-98 to 8.4% in 2007-10, and from 11.1% to 19.9% for those aged 15-54.
The proportion of black patients also increased from 16.6% to 25.6% during the study period.
The researchers, whose findings are published in the medical journal Stroke, say the ethnic disparities may be because of different cardiovascular risk factors.
"We observed a higher prevalence of hypertension and diabetes mellitus in black patients compared with white patients in each of the four time periods in all age groups," they say.
"Other possible explanations for ethnic disparities include cultural differences in perceptions of health and the health care system, environmental exposures, genetic factors, socioeconomic status, and educational attainment."
The increased risk for younger people could be because of a rise in classical cardiovascular risk factors such as diabetes mellitus, obesity, and high cholesterol level.
The decline in stroke incidence overall may be partly because of improvements in prevention, combined with an increase in healthy living. Drugs to lower cholesterol could also be a factor.
"We do not have information on the use of these medications among different age groups. However... the elderly might get preferential prevention treatments compared with young adults because stroke was not expected to occur in the young. Also, many young adults do not visit a doctor regularly to monitor their health status, especially with regard to vascular risk."
Dr Madina Kara, researcher at the Stroke Association, said in response to the study: "It's encouraging to see such a striking reduction in the number of people having a stroke in the past 16 years. Public health campaigns around the risk factors for stroke, such as high blood pressure and smoking are helping people to take control of their health and reduce their risk of stroke. This reduction, however, is not being mirrored in those under 45 years old, and the black population, where the incidence of stroke remains high.
"We know that the African-Caribbean community are at greater risk of sickle cell disease, diabetes and high blood pressure - conditions that can lead to stroke. This means they are twice as likely to have a stroke compared to the white population. In addition, haemorrhagic stroke, caused by bleeding within or around the brain, is more common in younger adults.
"Stroke changes lives in an instant and can have a devastating physical and emotional impact on not only the stroke survivor, but their family and carers as well. To help reduce stroke across the whole population, we all need to take steps to reduce our risk."