There is no afterlife darling, they're alive.
CTTO!!
Just my interpretation but as he was about die as an old man, He wanted to feel close to where Aka last was (hence going on the hike to the shed).
As he fades Aka snuck in and pushed a bit of his own flesh into his mouth ( you can seed blood on the fingers & mouth and a gulp).
S wakes up thinking he is in heaven, surprised even Aka talks, furthering his delusion that he’s in heaven. Only once Aka shows him his reflection and explains That eating demon flesh will give you immortality until I die, does he realize. (Aka fed him part of his body to keep S alive/allow them to be together For a long time).
For the sake of a happy ending I’d assume the demons power gave him regeneration so he has his nice young appearance back. They apparently are still living in modern times together and publishing a story, we see at the end.
CTTO!!
ITS ACTUALLYY GOOD ITS A BITTERSWEET ENDING,IN AFTERLIFE THEY ENDED UP TOGETHER AND LIVE HAPPILY EVER AFTER YEYYYY