Sheldon entered college at the age of eleven, right after completing the fifth grade.
At age twelve, filled with disappointment at not receiving a titanium centrifuge to separate radioactive isotopes, he had to be flown to a hospital by helicopter to be treated for radiation damage following a mishap when he built a CAT scanner (which also resulted in the death of his sister's guinea pig, Snowball).
He believes string theory better unites quantum mechanics with general relativity than does loop quantum gravity.
He neither gives credence to the notion that quantized spacetime will manifest itself as minute differences in the speed of light for different colors (as he expects matter consists of tiny strings) nor that only loop quantum gravity calculates the entropy of black holes.
Sheldon gets more serious in his relationship with Amy, even admitting that he loves her ("The Prom Equivalency"), and secretly revealing a family engagement ring ("The Commitment Determination").
Sheldon also creates a new scientific theory with Leonard and begins his studies on Dark Matter.
He also, at this time, conducted experiments involving the height of stairs only to deduce that if a step on a staircase is two millimeters off or more, a person is likely to trip - his findings evidenced by his father breaking his clavicle.
When Sheldon was thirteen, he attempted to build a nuclear reactor in order to provide free electricity for his town, but it failed after a government agent informed him that it was illegal to store Yellowcake uranium in a shed; he then had a big tantrum as a result.
Sheldon is characterized by a strict adherence to routine and hygiene; an overly intellectual personality; a tenuous understanding of irony, sarcasm and humor; and a general lack of humility or empathy, the former of which is demonstrated in the fact that he has no problem voicing to his peers his admiration for his superior intellect.
These qualities, along with his penchant for pranks, are his character's main source of humor.
Sheldon and Amy officially become boyfriend and girlfriend.
Sheldon eventually gives up on his research on String Theory and starts to pursue a different field.
None of his guests would come and the girls would torture him.