Jii-san has osteoporosis, he said his 'bone problems' began when he was '70', and yet he only asked to be 10 years younger when he reincarnated. That was very unwise. There's a whole world to explore and a lot of people to meet, but he chose to remain in a body that is going to have some medical complications very soon (as he is already aware). He can only hope that magic can fix or alleviate those problems.
I understand there is an abundance of middle-aged people being reincarnated into a very young age in the manga world, so this manga is reincarnating the geriatric main character nearly the same age before he died to retain some originality. That's understandable. I guess the author is the ruler of this world, and he is never going to mention the main character's health declining again: the whole 'old reincarnator' shtick was to stand out. It worked. I like it because he looks like a sage (and not a busty saintess or twenty-year-old guy that is just called a 'sage') , but it was still very unwise.
Is there a WN/LN?