The 'akaza sad story' might refer to the tragic backstory of Akaza from Demon Slayer. Akaza was once a human with a strong will. He had a family and a normal life, but due to various misfortunes and being turned into a demon against his will, he lost his humanity and was forced to do cruel things. He also had a lot of inner struggles as he still retained some of his human emotions deep down, which made his story a sad one.
Well, Akaza, originally Hakuji. He was a kind - hearted young man at first. He got involved with some bad people who took advantage of his situation. His love for the girl was pure, but they couldn't be together. When he became a demon, he lost some of his human memories but not all. His back story is full of pain and loss, which in a way, shapes his character as a demon. He is a complex character with a past that has both human and demonic elements intertwined.
One key element is his human life as Hakuji. His poverty and his attempts to care for his father are important. Another is his love for the girl, which was a major part of his human existence. And of course, his transformation into a demon due to the misdeeds of others towards him.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.