So a brief word on the age of Apocalypse.
The reason why various Marvel characters are dead is it's a different timeline. History was changed, and in this timeline basically everyone is dead. Big A has machines just to process the billions of humans his men slaughter. The global population is small and almost entirely mutant.
How did this happen?
Well the story starts as always with Professor Charles Xavier.
A visionary, a leader, and an inveterate horndog.
You might think a crippled guy in a wheelchair never gets laid, and you'd be wrong.
(Sidenote: I had a friend who drank and drove, until he wrapped his truck around a telephone pole. So of course as soon as he was out of the hospital I took him out drinking, and asked if his cock worked, I mean hell his spine and legs no longer did. Seemed reasonable. Well the answer is kinda. If he took a shitload of viagra it'd get hard enough for his GF to use him like a dildo, he said there was kinda a sensation, enough at least so he could cum. Now you know)
Professor X when he isn't dreaming of fucking his underage students actually slings cock all over the galaxy.
But today we are focusing on one particular union.
Gabrielle haller. Some more or less random chick. With whom he has a son (he's got several) named David Haller aka Legion. A batshit crazy schizophrenic. Dude has multiple personalities, which by itself no big deal, but each personality has it's own mutant power, and he has billions of personalities.
(Okay what the fuck? You ask. I thought mutant powers were genetic? Isn't that the whole point of Mr. Sinister and cable? Now its psychological? Fucking bullshit. Agreed dear reader, but that's the story)
And David doesn't see his dad much what with ProfX spending all his free time seducing studen-- I mean saving the worl-- errr well to be honest mostly just arguing with his old buddy Eric Lehnsher aka Magneto. And one day Davie boy gets sick of it. So he does the reasonable thing. He travels back in time to kill magneto so his dad will have the time to take him fishing.
David being the horrible fuckup that he is fails. And the manner in which he fails is he kills his dad.
About ten years before the founding of the X-Men a youngish Xavier gets his ass mercked by a random crazed mutant, and dies in magnetos arms.
So what's Eric do? Well he carries his friends vision forward on his own of course. What a good pal. (In timelines where magneto dies prof x does NOT found the brotherhood of evil mutants and carry Eric's dream on. Because he's a shit friend, father, and teacher)
And without the Professor and his telepathic powers to lead them, the new X-Men never find Apocalypse chilling in his alien starship. (Originally the first mutant, a title he shares with Namor, Apocalypse was born a pharaoh in ancient Egypt, and that would have been the end but he also found an alien starship and it keeps him alive to present day)
So Apocalypse starts his attack on Earth ten years earlier, and wins. Avengers crushed. Magnetos team fleeing for their lives. The world is his.
And this is the age of Apocalypse.