Some causes of dilated pupils in teenagers are head trauma, drug use or abuse, and low or insufficient light in the environment, according to MedicineNet.com. Dilated pupils are also medically known as mydriasis.
It is normal for the pupil to dilate, or enlarge, when exposed to darkness, notes Healthgrades.com. Some prescription medications and illicit drugs can also dilate the pupils, but the effect typically wears off in a short time.
Dilated pupils can be a symptom of traumatic brain injury, according to the American Association of Neurological Surgeons. If only one pupil is dilated following a head injury, it may indicate the presence of a mass lesion on the same side of the brain as the dilated, or blown, pupil.