Clark would be 28-30, Jimmy would be 23. I think that puts them close enough in age to be drinking buddies. Clark refers to him as Jim, but everyone else calls him Jimmy, a nickname that really pisses him off, but which he only confides pisses him off to Clark. The editors would have to watch like hawks to make sure that Clark/Superman never calls him Jimmy, and no one else ever calls him Jim (In fact, if they ever did a doppelganger storyline, Jim could say he started to get suspicious because the fake Clark/Superman called him Jimmy and not Jim).
One way to do this while preserving the idea that Clark is very entrenched in the business and Jimmy is the greenhorn would be to preserve the typical history of Clark that has him come to Meteopolis straight out of high school, so he's got 10-12 years in the newspaper biz and is an established investigative reporter (Superman is a great anonymous source
He's known for his hard hitting investigations into slumlords, big banks, and big corporations, generally advocating for workers and social justice by implication- and it is an investigation into Lex Corps that triggers a rivalry with Lex Luthor on multiple fronts), while Jimmy did go to college, which he kind of barely got through while goofing off and going to frat parties (His parents were big donors), possibly with a degree in photography rather than journalism, and that makes him a rookie newspaper photographer at 23 (Clark at that age would have been 5 years in since he didn't go to college).
Jim is sort of the wacky womanizing hard partying friend and Clark is the more responsible of the pair, but I think I'd make Jim a tad more mature than usual and Clark a tad less uptight than usual. Clark will have a beer or two, but Jim is the one getting him to the bar, staying after Clark goes to bed, and claiming to have hooked up with a girl and come straight from her place to the paper the next day, which Clark is generally good naturedly mock dubious about.
Jim is the first person Clark reveals his secret identity to. Ironically, Jim figures out Clark is in love with Lois, but Clark won't admit that to either Jim, Lois, or himself for a while.
Gradually, towards the end of the cycle between reboots, Jim gets pulled into the orbit of Clark's superhero friends, with the two of them hanging out with Bruce Wayne and other Justice League characters and Jim knowing most of their superhero identities, etc. and Jim winds up the best man at Lois and Clark's wedding (Which is about a year before the next reboot).
The closest I've seen to this was the New52, but it wasn't really quite the ideal dynamic I describe, and not consistsntly.
Another thing that they could consider, if Kara is in her 20s, is having Jimmy and her date and Clark NOT approve because he knows in detail about Jim's attitudes toward and history with women, and is protective of his cousin. However, the swerve would be that Jim turns out to be in love and committed and Clark sees Kara not taking the relationship seriously, flirting with other guys, etc., and has to have a talk with *her* about being considerate of Jim's feelings, despite initially thinking he'd be having that talk with Jim about Kara's feelings. Then you see where you want to go with that- I am guessing Kara dumps Jim and Jim's crushed, but it would depend on how it goes over with fans in the long run after they get used to the idea. Maybe Kara dumps him, he's devastated but eventually bounces back, and years later right before the next reboot, Kara does a long winded apology and talks about how wrong she was and how she's grown to see how much she meant to him and how much he offered her that others did not (made her smile, whatever), and asks for a second chance, and then they are dating again with an implication that it will lead to marriage eventually that second time around ("Hey, Clark, you never know, someday we could be brothers in law", cue Clark groaning, and Jim doing the trademark Superman wink at rhe readers), but with the big reboot happening within a few months after they get back together, so it never actually gets to a proposal or a marriage. Lucy Lane is someone Jim dates at some point, but they don't click and wind up just friends pretty quickly without much drama.