In such a fanfiction, Naruto's character arc would be completely different. He would be exposed to more complex and dangerous situations much earlier. His view of the world would be more mature from the start. His relationships with other characters like Sakura and Shikamaru would also be different. Since he'd be in a different age group, they might not have the same kind of bonds as in the original story. And his journey to become Hokage would start with a different set of goals and challenges, more related to the issues that were relevant to Itachi's time.
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.
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 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.
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.
They could be friends. They would grow up sharing their dreams and goals. Maybe they'd train together and push each other to be better ninjas.
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.
If Naruto and Mikoto were the same age, the relationship between the Uzumaki and Uchiha clans might be different. They could be more collaborative as Naruto and Mikoto's friendship would bridge the gap between the two clans. It might also change Naruto's relationship with Sasuke. Since Mikoto is Sasuke's mother, Naruto's bond with Mikoto could lead to a different kind of understanding between him and Sasuke from an earlier stage.
The ninja way as it is known would be completely overturned. Naruto has always represented values like friendship, hard work, and never giving up. As Sauron, he would promote values of domination and power over others. Shinobi training might change to focus more on dark arts and subjugation. Younger ninjas would be influenced by this new Naruto, and the future generations of the ninja world would be shaped in a very different and darker way.
It would be a huge shock to the Naruto universe. Konoha would be in a state of chaos. His friends like Sasuke, Sakura, and Kakashi would be devastated. They would likely seek revenge against the Akatsuki. The power balance in the ninja world would also shift. With Naruto gone, the threat of the Nine - Tails being used as a weapon would disappear, but at the same time, the Akatsuki would gain more influence and power, causing other villages to be on high alert.
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.