It's about that super fateful moment in HP when Harry Potter uses the resurrection stone for when he comes up agains Lordy Voldy for the last time. Haz sees the ghosts of his parents and freshly knocked-off (sad!) uncle Sirius Black and Professor Lupin. So much emotion, right?!
Prinsonersirius had a thought, though, that the ghosts of Harry's rents are wearing the same threads as they were when they were killed, but Sirius and Lupin are looking younger and much more fresh for a couple'a dead ghosts. It kind of suggests that though they were living longer than Harry's parents and so should look older, they actually are about the same age from when Harry was living with them which suggests a part of their soul and what them, them died alongside Lily and James Potter those years ago. Deep, huh?
Either way, we're still dev that everyone has to die and these guys just don't get to relax in the afterlife!