Jimmy being 10 whole years younger than Steph isn't a huge problem (he *could* be 24 in 2016 when he first appears) as the fact he and Kimmy seem to have a lot of common things in how they grew up, especially as they talk about the Gibbler home. However, it's also true that people phriase it the same way if they're talking about, say, colelge traditions, or how a certain teacher dols them such corny jokes, etc..
Admittedly, I'm leaning toward Jimmy being born in 1992, even though it's 15 years after Kimmy and almost 20 after Garfth, just for the line I can put in the Chronology:
Kimmy: "I knew something big was up the minute I heard my mom say, "Oh yeah, that's how that happens, isn't it?"
I mean, talk abut typical Gibbler.
Plus, 1992 actually works very well. We have the final 2 episodes over weeks of time as Kimmy and D.J. hope to go to Spain, then a whole summer where all you see is them coming back. Plenty of room for them to have Jimmy and Kimmy's never home anyway. (THe end of season 6 is also possible - only 2 episodes have to take place after the Valentine's Day week anniversary and stuff, and then it's clear except for the camp episode till late August/early September. However, while there's more time, the later it's pushed the harder it is to see Jimmy that young.)
STeph could see him as just a little kid if he's a few years younger and they're in school together - but while it seems odd for Jimmy to have a crush on a girl 10 years older, this is a Gibbler, and his definition of "cxrush" might be different based on what he'd been told as a kid, even. (I mean, this is the same kid who thought creamed spinach was ice cream.
)Theother option, JImmy as a cousin, works okay, too - while my comical "exit the petition" might lose out if hewas brought there to live at about age 5, that can still be in 1992 and maybe 1993 and let him be about 30, which I pictured him as at first. And, whule the Gibblers don't necessarily seem like the kind to adopt - social workers would frown on how little attention Kimmy got and worry about the same thing - nothing says Jimmy can't be left there with Kimmy's parents as legal guardian and just come to know the parents as Mom and Dad.