From Why Time Seems to Slow Down in Emergencies, a 2007 article by Charles Q. Choi:
When a person is scared, a brain area called the amygdala becomes more active, laying down an extra set of memories that go along with those normally taken care of by other parts of the brain.
"In this way, frightening events are associated with richer and denser memories," [researcher and neuroscientist David] Eagleman explained. "And the more memory you have of an event, the longer you believe it took."
Eagleman added this illusion "is related to the phenomenon that time seems to speed up as you grow older. When you're a child, you lay down rich memories for all your experiences; when you're older, you've seen it all before and lay down fewer memories. Therefore, when a child looks back at the end of a summer, it seems to have lasted forever; adults think it zoomed by...