Jay Dixit in Psychology Today:
Beginning in our late teenage years and early 20s, we develop and internalize a broad, autobiographical narrative about our lives, spelling out who we were, are, and might be in the future, says Dan McAdams, a psychologist at Northwestern and author of The Redemptive Self: Stories Americans Live By. The story is peppered with key scenes—high points, low points, and turning points—and a first experience can be any of these. “These experiences give us natural ways to divide up the stories of our lives—episodic markers that help us make sense of how our life has developed over time,” McAdams explains.
Part of why firsts affect us so powerfully is that they're seared into our psyches with a vividness and clarity that doesn't fade as other memories do. You may not remember the 4th real kiss you ever had, or the 20th—but you almost certainly remember your first. This is known as the primacy effect.
When people are asked to recall memories from college, 25 percent of what they come up with draws from the first two or three months of their freshman year, says David Pillemer, a psychologist at the University of New Hampshire. What people remember most vividly are events like saying goodbye to their parents, meeting their roommates for the first time, and their first college class. In fact, when psychologists ask older people to recall the events of their lives, the ones they most often name are those that occurred in their late teens and early 20s.