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 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.
One possible plot twist could be that Naruto, being at Itachi's age, has already mastered some of the more advanced jutsu much earlier. For example, he might have learned the Sharingan - like abilities through some unique encounter, which would completely change the power dynamics in the Hidden Leaf. This would also lead to a different relationship with Sasuke, as Naruto would be more of an equal in terms of power and mystery.
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.
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.
One interesting fanfiction could be 'Parallel Lives'. In this story, Sakura and Itachi being the same age creates a whole new dynamic in the Naruto universe. Their interactions are different from the original storyline as they are peers. For example, Sakura might be more involved in Itachi's early decisions, and Itachi could have a different view on Sakura's potential as a ninja.
A possible plot twist could be that Sakura discovers Itachi's true intentions about the Uchiha Clan massacre much earlier. Since they are the same age, she might be in a better position to understand his motives or even try to prevent it. Another twist could be that Itachi sees Sakura as a potential threat to his plans because of her intelligence and strong will, which could lead to an unexpected conflict between them.
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.