When the twins are five, they go to kindergarten and they get asked all sorts of questions, like if they can read each others' minds,
"No, but that'd be so cool if we could!" Mabel grins, latching on to Dipper's arm.
And why Dipper's name is Dipper,
"Because I like it better then my other one." Dipper says, and Mabel laughs because she knows Dipper's other name and thinks it's the funniest thing in the world.
And why Dipper has a constellation on her head.
"I dunno." Dipper shrugs, pulling her bangs down over the offending birthmark.
When the twins are six, Dipper asks his family to stop calling him by his "girl name". He explains to the best of his six-year-old ability that he isn't actually a girl. Dipper's parents try to pretend he’s just playing a game, and then try to tell him it's just a phase, but when eight months have gone by and the boy is wilting and wasting away under dresses and lamb costumes, Mrs. Pines goes out and brings home a stack of books with titles Dipper can't read quite yet.
Dipper doesn't hear them call him by the wrong words again, after that, and Mabel calls all the dresses in his closet that she likes. Their dad gives Dipper an old green baseball cap that sits too big on the boy's head, and Aunt Jessica gives Dipper a short haircut because Aunt Jessica does everyone's haircuts so that they won’t have to pay for a barbershop, and Dipper couldn't be happier.
When the twins are eight their Grunkle Stan comes all the way from Oregon for Chanukah, and it's their first time meeting him. The last time he bothered "wasting" his money on a plane ticket was when Mabel and Dipper were three, and they don't remember any of it, so obviously that time doesn't really count.
Grunkle Stan is loud and a little bit mean and he keeps a knife in his shoe, but he doesn't call Dipper by the wrong name at any point during the holidays, and he teaches both the twins how to cheat at card games. The twins decide they like him well enough.
They're ten when Dipper learns how to make his voice sound a tiny little bit lower. Mabel gets into the habit of making hers just the smallest bit higher, so there's more of a difference, and Dipper knows he's got the best twin ever. (Even if she teases him about his lack of manliness, sometimes.)
When they’re eleven, Mabel cheerfully coins the term ‘unidentical twins’ and Dipper can’t deny it fits. He never uses it himself, of course, but Mabel introduces them to new people and grins at their confusion and condescending replies.
“Hi! I’m Mabel. This is Dipper. He’s my unidentical twin.” Mabel gestures to where Dipper stands awkwardly next to her, and he waves.
“Don’t you mean fraternal, sweetie?” Their grandmother’s friend asks, her smile slightly strained, and Mabel’s own smile only grows.
At age twelve, a lot happens, and it's all journals and demons and secret societies. At age twelve, Wendy happens, but Dipper eventually tells her how he feels and everything works out pretty much okay. At age twelve, Bill possesses his body and Dipper expects a comment, or a reassessing glance, but Bill just gets right to throwing himself down the stairs and stabbing himself with forks, like he doesn't even notice any difference. And maybe he doesn't; Dipper has no idea. But despite the situation he actually feels relieved when Bill doesn't say anything.
They're thirteen when puberty starts to hit, almost fourteen when Dipper takes to wearing two sports bras over each other because he can't afford a real binder, and everyone online says bandages are a no. They're also thirteen when their cycles start, and that summer the twins scare Soos many a time with the short tempers brought on by cramps and hormones. Soos, being the great friend he is, says nothing, and buys extra chocolate bars and gummy koalas when they're having trouble.
Once or twice he invites Dipper over for "boys' night", which is basically just making fun of old movies while Soos' abuela attempts to vacuum the entire house for the ninth time in a day. Wendy brings Mabel to her house for movies too, on those days, until the day soon comes when both parties get fed up with the background noise, (Wendy's brothers are always home and always yelling) and the four of them continue movie night together at the Mystery Shack. It’s actually more fun that way.
The twins are fifteen when they get a package in the mail for Chanukah, from Grunkle Stan. Inside is a picture of Stan, Wendy, Soos, Waddles, and Gompers, and a half dozen skeins of the sparkliest yarn Mabel's ever seen. Underneath those is a binder, and a note saying that it'd better be the right size, because "I actually bought the thing new, at full price. Do you know how much these things cost, kid? It better fit."
It doesn't fit, it's a size too big, but Dipper keeps it because he'll probably grow into it eventually.
And everything’s okay.