Asta's age in the Black Clover story arc changes over time. At the beginning, he's around 15 years old, and as the story goes on, he gets older. The exact age depends on the particular events and chapters.
One way Asta could turn evil in a fanfiction is if he was constantly ostracized for his lack of magic in a more extreme way. For example, if the Magic Knights not only rejected him but actively bullied him. This could lead to a build - up of anger and resentment. Eventually, he might decide that the system that has shunned him is wrong and needs to be overthrown, leading him down an 'evil' path of using his anti - magic to cause chaos rather than protect.
One idea could be a story where Asta and Vanessa go on a special mission together. They face a powerful enemy and have to rely on each other's unique abilities. Asta's anti - magic sword combined with Vanessa's thread magic could create some really cool battle scenes.
A potential plot could be Asta's slow descent into darkness. He starts off as a bit more cynical after facing continuous rejections. Then, he meets a group of outcasts who have also been wronged by the system. They convince him that the only way to make things right is to take power by force. Asta begins to use his anti - magic to steal magic from other mages, believing it's a way to level the playing field. As he does this, he becomes more and more feared and is seen as a threat to the kingdom.
They could embark on an adventure to find a hidden magical artifact. This artifact could be said to have the power to enhance one's magic abilities even further. Asta would be eager to get it to become stronger and prove himself, while Vanessa would be more cautious but still go along to support her friend.
One common theme is power. Since Asta has a demon, many fanfics explore how he gains and controls new powers. Another theme is friendship. They often show how Asta's relationship with his friends is affected by his demon side. For example, some fanfics have his friends being initially scared but then coming to accept him.
One great 'Black Clover Asta x Rebecca' fanfic is 'Asta's Promise to Rebecca'. It beautifully explores their relationship in a world full of magic and adventure. The author really captures Asta's determination and how Rebecca sees him in a new light.