In House of X #4 on a mission in space a whole team of X-Men are killed (wolverine, jean grey, cyclops, nightcrawler, mystique, archangel, Monet and Husk).
In House of X #5 we learn Prof. X has a plan in case any x-men member (any mutant) is killed. We learn the mutant Goldballs' gold balls are actually eggs. Then another mutant makes the non vital ones...vital via realit warping. Another mutant causes cell repoduction inside eggs, a time manipulating mutant ages the embreyos while Hope Summers ensures the success with her psychic powers (they are known as the 5).
Prof x using cerebro uploads a copy of each of the dead mutants' memories, personalities, etc into the eggs.
When the eggs hatch the killed x-men are reborn and fully their selves. I believe same age as they died through the aging process in the eggs.