In “The Last Problem” Luna mentioned “Silver Shoals” - a place where she and Celestia will go.
After searching for information about this place, I found out that Silver Shoals is a retirement community.
Luna also asks Twilight to visit them there, which suggests that sisters will remain there indefinitely, possibly forever.
From “The Journal of the Two Sisters” book it’s clear that the alicorns do not age due to the use of a large amount of magic. Sisters used strong magic to interact with the heavenly bodies.
After Twilight took Celestia and Luna’s place, two sisters stopped using such strong magic. Most likely, they will become old as usual ponies…
That’s sad, but that’s what creators of the series decided.
I tried to imagine Luna in old age. I think she is still beautiful like before.