Will a single head injury have lifelong effects?

It depends: Head injuries vary in severity from minor grade 1 concussions to grade 3 concussions to major head trauma that can cause brain hemorrhage or diffuse brain injury. Repeated concussions can have lifelong effects. However, the data is not there yet showing that a single minor concussion will have lifelong effects.

Depends on severity.: A moderate or severe head injury can have lifelong effects, with the greatest amount of recovery happening within the first year after the injury. A mild injury (about 75-80% of tbis) are expected to make a nearly full recovery within 3 months after the injury; although, 6 months after the injury about 28% of people report having continuing mood problems, headaches, dizziness, and insomnia.