Disclaimer: Well, it's so weird I doubt even Whedon could've come up with it, much less Kripke, so I'm gonna go ahead and say I don't own Supernatural.
Bruce Springsteen's 'Atlantic City' was kind of my soundtrack writing this (and the italicized bits are lyrics from said song), if you want some ambiance for your reading. Again, this is all majorly AU with a very strange premise, and there's character deaths and angst lying around like potholes on the Beltway, so consider yourself notified. I hope you like it.
Well now, everything dies, baby, that's a fact
But maybe everything that dies someday comes back…
Dean is turning thirty and Sam is halfway through twenty-six when it occurs to Bobby, who's hosting the little birthday party (which consists mainly of three six-packs and ribs on the barbecue) that Sam has a bit of a baby-face, for someone who's now closer to the dreaded 3-0 than his roaring twenties. Sam wings him with an empty can and Dean makes fun of 'widdle Sammy' and the evening ends with all three of them drunk and singing Springsteen on the roof.
Dean is thirty-four and Sam is celebrating his thirtieth when, again, Bobby makes the same remark. This time there's a note of disbelief; they haven't seen him in person for almost a year, and he whistles, looks Sam up and down, and swears, "Boy, you haven't aged a day."
Dean is thirty-nine and Sam thirty-five when they finally acknowledge there's a problem. Dean's hair is beginning to gray at the temples, and there are lines sinking in around his eyes, but Sam is still getting carded once in a while. He looks twenty-four on a good day, and they start looking back in the journal for a cause- trickster, demons, witches, anything. And come up completely blank.
Dean is forty, unbelievably, and Sam is thirty-six the first time they bump the age on a fake ID down, rather than up. In Mariposa, California, they're called Agents John Deacon and Brian May (apparently no one around here listens to Skynyrd, Dean bitches, so they probably deserve the hauntings) and their badges say Deacon is forty and May twenty-seven. They still have no idea what caused this, but it's not exactly their biggest problem when you consider the spirits in Virginia, the ghoul nest waiting in Utah, a possible werewolf across the Canadian border…
Dean is forty-two and Sam thirty-eight the first time someone mistakes their relationship. They walk into the little diner, and Sam's just taken one to the head from a seriously pissed off wraith, so Dean has his hand on his brother's shoulder to keep him walking in a straight line. The waitress hands them menus and after getting Dean's order asks "And what'll your son have?" They both very conspicuously choke on their water.