It's not sudden, until it is.
Kurt's known his whole life, or it feels like it, but won't make it past thirty always felt so unreal when he wasn't even sure he'd be able to survive high school.
Tony at twenty-five, married by thirty. It's not a schedule. It's a deadline.
Blaine honestly doesn't realize what it means for Kurt, for them, for a very long time. Years, actually. The fact that they have years is what keeps him from that realization. Thirty seems so far away when you're seventeen and in love. Or nineteen and moving into a shoebox in New York with your boyfriend and his best friend. Or twenty-three and getting married. Or even twenty-seven and eating pineapple on a beach in Hawaii for your 10th anniversary, trading sticky sweet kisses as the setting sun makes you both glow like gods.
It seems a lot closer when you're twenty-nine and realizing that your husband is baking cakes every few days because neither of you are sure he'll live to eat your next anniversary cake.
Blaine comes home from work and finds Kurt crying into the cake batter.
"I'm never going to get to be fat Blaine. God, that seems so stupid, but I'm never going to get to be fat and old and wrinkly."
"It's not stupid." Blaine kisses him to make him stop talking, and for a second he's seventeen and stupidly in love and sure of the future all over again. Then Kurt pulls back and runs to the bathroom to puke and he's so so old again.
He starts measuring Kurt's moods by the cakes he makes. The more complicated the concoction, the worse his day went. The more scared he is. Blaine doesn't like banana bread very much, although it's Cooper's favorite, but he starts to look forward to the banana bread days with a desperate hunger that have nothing to do with the food.
Blaine jokes all the time about how both Kurt and the desserts he makes are both 'delicious and ba~ad for you' while waggling his eyebrows outrageously until Kurt throws batter at him or tackles him to the ground so they can have sex on the kitchen floor. Or both. One time Kurt was making a profiterole pile and they ended up licking the melted chocolate off each other.
They go to a revival of RENT for Kurt's 30th birthday, giggling and throwing popcorn at each other, and Kurt feels positively giddy with love. It's like they're teenagers all over again, only without the awkward lusting on Kurt's side and infuriating obliviousness on Blaine's.
They mouth along to the songs, Blaine pulling exaggerated faces and Kurt smothering his laughter into the silk gloves Rachel gave him as a birthday present.
Kurt coos over Angel's fabulousness, and Blaine sighs over Roger's biceps. They both agree that Kurt should help Mark go scarf shopping, because those colors do nothing for them. They don't even care that they're getting a couple dirty looks for their constant low-level thrum of chatter because they're both absolutely drunk with love.
And then-
One song
Before I go
Glory
One song to leave behind
Find one song
One last refrain
Glory
From the pretty boy front man
Who wasted opportunity.
And Blaine bursts into tears. Not quietly either. Huge, gasping sobs that rack his entire body and send him shuddering into spasms of grief and pain.
The actor playing Roger is actually shocked into silence. Everyone in the theater is staring at them, open mouthed, as Blaine lets out high, keening sounds that are inhuman in their agony.
Kurt is mortified.
He drags Blaine back home, his husband a sobbing, miserable wreck the entire taxi ride.
They both scream at each other. Blaine pulls his hair until chunks come out and Kurt throws his favorite antique gravy bowl against a wall.
"What do you want me to do? Not be upset that you're fucking dying?"
"I want you to not fucking humiliate me in public like that and ruin the time I have left! I am the one who's dying Blaine, not you!"
"Yeah, because it's so fucking easy seeing you like this!"
"Seeing me like what Blaine? Throwing up? Tired? Why don't you fucking leave if it's so hard seeing me like this then!"
"You're the one who's leaving!"
"It's not like I want to Blaine! I can't fucking stop myself from dying! Don't you think I fucking would if I could? Oh so, sorry, I never considering that, not dying. Let me just get on that!"
"Don't turn that around on me like that, you know-"
"I know? I know what Blaine? I'm not fucking leaving you."
"Yes you are. And I'm going to be the one who has to try to keep on living after you die. How the fuck am I even supposed to do that?" The question comes out much more quietly than anything else that's been screamed across the room. It's soft and scared, and he looks at Kurt with wide red eyes as if honestly asking him to tell Blaine how to do it.
"I don't know. You'll have to figure it out." Kurt turns away, suddenly tired.
He's so very tired.
"I don't know how I'm supposed to survive without you."
"Well, if I see you in the afterlife or whatever anytime before a good 60-70 years have passed I'm going to kick you straight out and back to Earth. So don't get any ideas."
Kurt's voice is snappy, his shoulders hard lines underneath the soft grey of the designer sweater Blaine bought him with his first real paycheck.
Blaine wants to claw out his own heart and offer it up in exchange for this beautiful boy's life. Kurt is so bright. He lights up the world.
Blaine's so scared of going back into the dark.
Kurt goes to bed, and Blaine just stares at where he was standing for another hour or so. Tries to remember the way every hair curled at he back of his neck, the slight twitchings of his knees as he stood, the way he drummed Teenage Dream on his arms the way he always does when ever he gets upset. Blaine could stand there for the rest of his life, and he still wouldn't be able to remember all the little beautiful things that make up Kurt Elizabeth Hummel.
So he crawls into bed, and they make love silently as Blaine presses wet open mouthed kisses against Kurt's neck and Kurt grips his arms so tight he leaves marks. Blaine wishes he could tattoo those marks into his skin, and bites Kurt's shoulder as he enters him to stop from begging.