The story would be transformed in many ways. Naruto, being as old as Itachi, would have had different training experiences. He might have been trained by different people or in different ways. This would lead to different jutsu and abilities. In the political arena, Naruto could have played a more active role in preventing the Uchiha - Leaf Village rift. His relationship with characters like Kakashi and Jiraiya would also be different. For instance, he might have been more of an equal to Kakashi, and Jiraiya might have had different expectations of him. All these changes would create a whole new narrative for the Naruto story.
If Naruto was the same age as Itachi, their relationship could be complex. They might start as rivals, both being extremely talented in their own ways. Naruto's boisterous and determined nature could contrast with Itachi's more reserved and burdened self. Itachi might see in Naruto a different path for the Uchiha, one that doesn't involve the darkness he has known. Naruto, on the other hand, would probably be in awe of Itachi's power and at the same time try to understand his seemingly cold exterior.
Their relationship would likely become more of a friendly rivalry. Since they are the same age, they would constantly be comparing their progress in ninjutsu and other skills. Naruto would probably be more eager to prove himself against Itachi, and Itachi might see Naruto as an interesting opponent.
If Naruto was born the same age as Itachi in fanfiction, it would change the power balance in the village. Naruto could be part of the decision - making processes earlier. His influence on the Uchiha clan's fate might be significant. He could use his unique ability to persuade and his strong will to try and bridge the gap between the Uchiha and the village. Also, his interactions with Kakashi would be different. Instead of Kakashi being an older, more experienced mentor, they could be more like comrades or even rivals in some aspects.
In many Naruto fanfictions, the ages of Naruto and Itachi can be quite flexible. Sometimes, authors might change their ages to create different story dynamics. For example, an author could make Itachi younger to explore a different version of his character development, perhaps showing him still in the early stages of his ninja training while dealing with the dark secrets of the Uchiha clan. Naruto's age might also be adjusted, like making him older to be in a more equal power dynamic with Itachi from the start.
If Naruto was born the same age as Itachi, the story would be drastically different. Naruto would be more mature and skilled earlier. He might have been involved in the Uchiha massacre situation. Maybe he could have even tried to prevent it, given his nature of not wanting to see people suffer. This would also change his relationship with Sasuke. Instead of being an older peer, he would be more of an equal in age, and their rivalry could take on a different dynamic, perhaps more of a mutual understanding from the start.
One interesting plot twist could be that since they are the same age, they become rivals not just in power but also in terms of winning the affection of their peers. Naruto, with his boisterous and friendly nature, and Itachi, with his cool and mysterious aura, both vying for the same friends. Another could be that they end up in the same ninja team, which is unheard of in the original story. This would force them to work together and learn from each other's strengths, like Naruto teaching Itachi about the power of friendship and Itachi teaching Naruto more refined ninjutsu techniques.
One possible plotline could be that they become closer friends early on. Since they are the same age, they might train together more often. They could also share secrets that in the original story they wouldn't due to the age gap. For example, they might both have dreams about the future of the village that they can discuss on an equal level.
Their relationship with Sasuke would be very different. Naruto might not look up to Itachi as much as in the original story, and Itachi might not see Naruto as a potential threat to Sasuke's growth. Instead, they could both be like older brothers to Sasuke, guiding him in different ways.
One possible scenario could be that Naruto and Mikoto grow up together in a different Konoha. They might become friends and train together. Naruto could learn a lot from Mikoto, who is from the Uchiha clan and has her own unique skills. Maybe they would be in the same class at the ninja academy, and Naruto's positive attitude would influence Mikoto as well.