The strangest superheroes of all have been around for about fifty years at this point, but there are many mutant members of the X-Men whose history stretches back way, way further than that.
For some, the mutant gene manifests itself in being able to fly, or shoot laser beams from their eyes, or have their skin turn invisible so you can see all their insides (which is pretty gross). For others, their principal superhuman ability seems to be being old. Like, so old. Old as balls. Wolverine's pretty cool with his knife hands and mutton chops, right? Yeah, he's super old. He's your grandfather times your great grandfather. Just a bit less racist and much better looking in yellow spandex.
In reality Marvel Comics has a history that begins around the time of the Second World War, when a group of superheroes called The Invaders banded together to fight Hitler. In the fictional Marvel Universe that's also around about the time metahumans showed up, so far as most people knew. In fact the homo superior leap in evolution actually happened decades, even centuries earlier - unbeknownst to everyone. Including those early writers.
Mutants were around way before the X-Men showed up to "lead" them, when fancying Jean Grey was just a creepy glimmer in Charles Xavier's eye. Some mutants were part of that initial wave of WWII-era heroes; some have thought in even more wars than that; and some have been around for something like 2000 years. And they're still going strong. Good genes, we hear.
These are the ten oldest X-Men in the comic book's history...