Everybody thinks this story is stupid. I don't exactly see why. It makes perfect sense that lifespans are longer in the far future; it also explains why the Legionnaires are still called "Boy" and "Girl" in their 20s.
People seem to be disturbed at the idea of the Legion treating Superboy to wipe this information from his mind. Yet it was established early on that Superboy (and Supergirl) are always hypnotized so they don't remember anything that would reveal details of their futures. How this is any different, I simply don't know.
Okay, it's not the best Legion story out there...but it's far from the worst.