Oh, so we are going the romance route? Perhaps I should add a tag...
Normally I get very wary of romantic engagements with reverse-appearance age gaps like this (as it's so often an author's thinly-veiled excuse in setting up some unsavory fan-service) but this one appears to be ludicrously softhearted and rather wholesome so I'm willing to give it a free pass on that count for once.
So will their relationship end up like Sokuochi Lolibabaa, or like Asakura Hakase no Bibouroku?
@greatninja3
You should read the things I mentioned, are two different takes on immortal girl and their mortal man.
More detail in the first one there is
reincarnation, guy dies of old age but he finds her again in the next life and they continue their relationship.
Second one
she gets pregnant and their daughter inherited the immortality and her aging slowed as she got closer to the same apparent age as her mom, meaning that while he died, he was able to give her someone that wont.