I see some issues with dividing Immortal cleanly from Elderly. Four villains come to mind that would be at issue if these two categories were divorced from each-other. Albert Simon, Ra's Al Ghul, Mystique and Mother Mayhem.
Simon is immortal unless killed, he can technically die he just has no natural life-span, he also was in his autumn years at the time he achieved this state so he permanently looks like a man in his seventies or so.
Ra's of-course has an immortality that is dependent on continued used of a rejuvenation treatment, if he ever stops he becomes mortal again, another fellow who had his share of grey hairs when he discovered this.
Then we have Mystique who is very old but cheats death by constantly shifting her genetic structure to remain young, she's old without being immortal, but with no hard expiration date.
Mayhem is a psychic who cheats death by taking over the bodies of others. Her normal method of attack is to turn folks into mind-slaves but with some time and enough concentration she can transfer her consciousness into one of them as her new permanent-ish body. She, or at-least her good-alternate reality version, still considers herself an elderly woman who just looks young, but this makes her both physically young and potentially immortal. Aside from that she is the shake her walking-stick at the young folk, type. There are other body-hoppers out there so this would by no means just a be a unique case.
(Note I am not talking about a ghost or demon who can exist without a body but rather someone who continues to live only because she has this technique)
The first one is hard to kill and long-lived but still able to be killed, the other three all need to maintain-upkeep to remain young/alive. Also the first two look elderly. How would you propose we labeled those four, or similar types of villains if we were to untangled Elderly from Immortal?