Leonard was born on May 17, 1980 in New Jersey to Alfred and Beverly Hofstadter (Same month that Mt. Helens' erupted and "The Empire Strikes Back" came out.). He has two siblings: a younger brother named Michael and an elder sister that we don't know the name of ("The Maternal Capacitance", S2E15).
Before he went to Princeton, his parents put him in advanced placement classes, and sent him to learn cello, "getting me beaten up enough", as he described himself ("The Hamburger Postulate", S1E5). Leonard had his first kiss at age 19 to a girl named Geraldine Coco.Leonard instantly falls in love with her, even inviting her to their apartment for lunch.