15% of all cases of ischemic strokes occur in patients less than 45 years old.
To put things into perspective, incidence of stroke in this age group is twice that of multiple sclerosis.
Delayed diagnosis is due to several factors:
The relative rarity of the diagnosis in comparison to stroke mimics at this age, the 3 most common being: migraines, seizures, and Bell's palsy.
Atypical presentations, such as acute vestibular syndrome.
Although “typical" risk factors (such as smoking, diabetes and hypertension) are present in young patients with strokes, other factors to be considered are high-risk alcohol consumption, cocaine use (especially smoked), physical inactivity, sleep 6 hours or less a night, and known thrombophilia.
References
Lo DW, Kumar R. Arterial Ischemic Stroke in Children and Young Adults. Continuum 2017; 23(1):158-180.
Singhal AB, Biller J, Elkind MS, et al. Recognition and management of stroke in young adults and adolescents. Neurology 2013;81(12):1089-1097.