Fans of the hit sitcom will know that Sheldon (Jim Parsons) had a first roommate, Sebastian (Oscar nominee Steven Yeun), prior to Leonard (Johnny Galecki) moving in.
It was the one titled The Staircase Implementation. He moved in with Sheldon in 2003.
The Staircase Implementation - The Big Theory (Season 3, Episode 22) - Apple TV.
Sheldon never met Leonard, Howard, and Raj until they were all around 24. Penny didn't move into the building until Sheldon and Leonard were both 27 and have been living there as roommates and best friends for three years; Penny was only 21 then.
Sheldon and Leonard live together out of convenience not for financial reasons. Sheldon has said in the past that he wouldn't have a roommate if he could afford the rent. Penny on the other hand who lives alone on a smaller wage, is seen always struggling to get by, living paycheck to paycheck.
A Roommate Agreement is a written contract between roommates that outlines their rights and obligations while living together. This agreement includes house rules, maintenance duties, restricted behaviors, and more. For a Roommate Agreement to be useful, everyone sharing the household must agree to it.
My take is that at the time when he was interviewing for a roommate, he was just freshly starting out at his job so the rent was too expensive for him to afford alone, but as his work in the university gradually secured him a steady income years later, he earned more than enough to live alone, but he simply chose not ...
"The Big Theory" The Romance Recalibration (TV Episode 2017) - IMDb.
Currently rewatching Season 4, Episode 21, where Priya finds loop holes in the roommate agreement to benefit Leonard for once, so Sheldon blackmails her into getting Leonard to sign a new agreement that better benefits him.
Do Not Use Each Other's Things Without Asking. Not using each other's things without asking is one of the more obvious roommate rules. While some people are more casual about sharing, others are not. You have to learn what kind of person your roommate is and then act ingly.