It's entirely possible that he is Sidious, even though that they look different in AOTC. Remember that the Emperor is a master of deception and if Palpatine is the Emperor, then he could be giving off the appearance of a younger look as Sidious. We can't see Sidious's entire face to know for certain. As for Palpatine seemingly aging rapidly, keep in mind that in ROTJ, the Emperor looked extremely old, with the wrinkled face and the seemingly false dependence on his cane. That would certainly chime in with Palpatine being the Emperor wrt to the aging issue.