OPTION ONE: The only thing different about your character is that they are old. They still maintain their memories of castle life and their life at home, but as far as they know, they've woken up in a very strange place and are not only very wrinkly, but also limited to the inevitable ailments of old age. I implore you to play this up like there's no tomorrow!
OPTION TWO: Your character is not only old, but also believes they have lived a long life and are now coming to the end of it in a remote retirement home. What went on in that life? What are they like now? As long as they believe they are residents of this nursing home and don't remember the castle or their homes, that is for you to decide. Have fun!
OPTION THREE: Old people not your thing? You are more than welcome to have your character opt out of the plot by becoming one of the faceless members of the staff.
But Elaine, what if my character is a troll/vampire/superhero/giant rabbit/non-human?
Do not fret, friends! This plot does not require you to change your character into a human if you don't want to! If they are a resident, the staff won't judge them no matter how non-human they look. However, if you want to change them into a human, I am totally fine with that. Just know that it is not a requirement.
Okay, you've gotten the character bullshit out of the way. What about the journals?
The journals will still work for characters in the plot, altered or not!
POWERS DO WE STILL HAVE POWERS
Aged-up residents will, but your abilities will be severely dampened due to your age, so say you could call down lightning from the sky? Now all you can do is give people a little static shock, maybe do some flashy sparks if you exert yourself. Staff will not have powers, but they won't exactly back down from a challenge! Think of them as highly dedicated trained nurses.
Remember guys, this plot has potential to be hilarious, but only if we all work together to make it hilarious!