Do you know what the manga I'm talking about? I've read it few years ago. I think it's already fully scanlated, but I forgot the title. I remembered some of the stories, though.
So one day someone (male or female, I forgot) came to our bullied, timid, and weak protagonist's apartment. So you see, our protagonist need to consumed a medicine frequently. If he don't, he will feeling sick. This guest banned our protagonist to drink his daily medicine. He suddenly looking pale and then fainted. When he regained consciousness, he's taller, noticeably older and stronger than ever before.
So, skipped to last chapter, our protagonist stopped consuming his medicine. He grew older and older and he became an old man in the end. It's not a side effects of not consuming the medicine, it's because he's actually an old man and consumed the medicine kept him to his young appearances. Sadly, he didn't remembered anything about his old life, and the memory only back to him again when he's old.
Do you know what manga/mabhwa I'm talking about?