There had been a 25 percent increase in the number of strokes among those aged between 20 and 64 over the past 20 years -- now making up 31 percent of the total compared to 25 percent before 1990, said the studies in The Lancet medical journal.
Under-75's now account for 62 percent of new strokes, 45 percent of deaths, and 72 percent of illness and disability.
The researchers found that more than 83,000 people aged 20 and younger suffered strokes every year -- about 0.5 percent of the total.
And they warned the shift in stroke burden towards younger people would continue unless urgent steps are taken to get people to improve unhealthy lifestyles -- mainly by reducing consumption of salt, calories, alcohol and tobacco.
They showed that in 2010, 61.5 percent of disability and 51.7 percent of deaths were caused by haemorrhagic strokes -- which occur when a weakened blood vessel ruptures and is mainly caused by high blood pressure and unhealthy lifestyles.
"Most affected are people younger than 75 years and those living in low-income and middle-income countries where incidence of haemorrhagic stroke has risen by around 19 percent," said a statement.
Haemorrhagic strokes were half as common as ischaemic strokes, which are caused by an obstruction within a blood vessel supplying blood to the brain.
The studies warned that disability, illness and death caused by stroke would more than double worldwide by 2030 -- especially in low and middle income countries.
"This is in part because of a rise in the prevalence of risk factors involving unhealthy diet, high blood pressure, obesity, physical inactivity, and smoking in these countries," said the statement.