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.
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.
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 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.
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.
His relationship with Sasuke would be very different. Instead of being a peer trying to bring Sasuke back, he'd be more like an older figure. He might be someone Sasuke looks up to or even fears because of his power and knowledge at that age. Naruto would also have a different relationship with Kakashi. He'd probably be more of an equal to Kakashi in terms of combat ability and strategic thinking.
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.
Naruto would likely become more stealthy. Itachi is known for his stealth and if he trains Naruto, Naruto would pick up some of those skills. He might start to rely less on his brash and straightforward fighting style and incorporate more subtlety.
The relationship between Naruto and the Uchiha clan would be different. In the original story, there was a lot of tension between Naruto's time and the Uchiha. But if he was the same age as Hiruzen, he might be in a position to prevent some of the tragedies that befell the Uchiha. He could be friends with the young Uchiha members at that time and work towards a more harmonious co - existence within the village.
In this fanfiction scenario, Minato would not have the same kind of paternal - like relationship with Naruto. Instead, they could be rivals in love for Kushina's affection. Minato would also have to prove himself not just to Kushina but also to Naruto. Their relationships would be a web of competition, friendship and budding romances. It would be a completely different story from the original, with different power dynamics and emotional entanglements among the three characters.