This is based somewhat on the movie Raising Helen. It is AU so there is no Roy, Jim doesn't have a roomate and doesn't really fit in anywhere in the show timeline.
Disclaimer: I don't own Pam or Jim or the other Office folks and anything associated with them.
Jim Halpert was a big kid. That was the best way to describe him. He loved to play jokes, goof off and just have a genuinely good time. He did of course do adult things, like his job, paying his bills and other such adult activities, but they weren’t really his number one priority.
Jim loved to make people feel comfortable and make sure they were having a good time, above all he loved making people smile. The one person that he loved made smile was his best friend Pam Beesly. Whether it was cracking a joke, playing a prank on Dwight or just talking to her Jim always found a way to get Pam to give him a smile.
So Jim was an adult in the sense that he was in his mid twenties and had certain adult responsibilities, but he still had a lot of growing up to do. Getting Pam to smile though isn’t how Jim Halpert finally became more of an adult. The story of how Jim Halpert grows up involves tragedy, love and two innocent children.