By its very premise, The Big Bang Theory was about unusual characters attempting to interact in everyday environments. All of The Big Bang Theory's characters, particularly its core cast of scientists and engineers, had their own neuroses and struggles with social norms, acting in ways that most people would consider odd.
One of the traits present in many characters was that they acted immature and distinctly younger than the late-twenties they began The Big Bang Theory as. Whether they were immature in their workplace behavior, relationship with their parents, or any number of other behaviors, numerous characters acted below their age.
6 Leonard Hofstadter's Facade Slipped Often
The role typically occupied by Leonard Hofstadter, particularly in The Big Bang's early seasons, was that of the most 'normal' of the four scientists and the closest to a 'regular' guy. Although similar in awkwardness and lack of social graces as the others, Leonard was always aware of his own deficiencies and those of his friends.
However, it was revealed that Leonard possessed as many neuroses as his friends. At times, his maturity would slip. He was easily baited into unnecessary arguments with others. He would also act pettily and childishly, especially since his difficult relationship with his mother often caused him to act overly needy or reckless when she was present. Developing less than many of the other characters, Leonard was one of the few characters to not mature much throughout The Big Bang.
5 Howard Wolowitz Began The Big Bang Theory Barely An Adult
In The Big Bang's first few seasons, Howard Wolowitz was unquestionably the least mature member of the cast. He still lived with his mother, who treated him like a child. He would be given chores, have her drive him places, and she often grounded him when he 'misbehaved'.
As The Big Bang went on, Howard underwent a huge amount of development to become one of the most mature of the main cast. Although his relationship with Bernadette often emphasized his childlike approach, Howard became a responsible father and husband. Howard is one of the members of the group furthest through major life milestones and a far more capable person than he started out as.
4 Amy Farrah Fowler Had Many Moments Of Childishness
A case with some characters in The Big Bang Theory was that they acted immaturely as adults because they lacked the opportunity for a real childhood, often due to difficult relationships with their parents. Such was the case with Amy Farrah Fowler, who grew up with a strict and overbearing mother who didn't allow her to have friends or enjoy herself.
Although Amy was by far the most mature in her relationship with Sheldon Cooper, around others she was more childish. She was liable to get overexcited over anything involving her friends, often acted in inappropriate ways with Penny and Bernadette, and acted as if she was making up for her childhood.
3 Raj Koothrappali Struggled With Independence
In The Big Bang's earliest seasons, much of the humor around Raj (that didn't involve his status as an Indian immigrant) focused around his selective mutism that left him unable to speak around women. As these elements became less prominent, Raj's personality was revealed to be far more immature than many suspected.
Although he was one of the more emotionally-engaged members of the group and comfortable with his own identity, Raj struggled with maturity in a few ways. For one, he lacked financial independence, having his parents subsidize his lifestyle and struggling to budget at all. In addition, he was prone to pettiness, over-emotionality, and irrational behavior. Although not the worst in The Big Bang, Raj did prove to be one of its less mature characters.
2 Barry Kripke Was A Petty Workplace Rival
In the handful of recurring staff at Caltech, Barry Kripke appeared in almost every season as a rival to Sheldon, and an occasional friend and frequent nuisance to the rest of the gang. His exact characterization often varied between episodes, but he was often shown to be immature.
Although undeniably smart, competent, and occasionally capable of being kind, Kripke spent most of his screentime being petty towards other characters. He frequently plays pranks, mocks people, or acts shallow and childish. Almost incapable of being professional to his peers at work, Kripke was one of the most immature characters in The Big Bang.
1 Sheldon Cooper Was Always The Least Adult Character
Sheldon Cooper was The Big Bang's most iconic character, primarily due to his distinctly unusual characterization. In contrast to the other characters who were quirky but largely functional, Sheldon was typically described as barely able to look after himself.
Whether it was being unwilling to transport himself to work, acting like a younger brother to the much-younger Penny, struggling to understand social norms, or finding his Darth Vader sheets scary, Sheldon frequently acted like a child. A great number of storylines had the other characters explicitly liken themselves to Sheldon's parents.