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Q: I was surprised that the movie this time painted Naruto and Hinata’s love story.
• *In my head, I had decided Naruto and Hinata to get together a bit after they became young adults.
• But I couldn’t think of an actual love story for the two and was not planning on drawing it.
• However, I wanted to circle back to the first chapter (of the series) by making Naruto have a son who graffities on the (now) Hokage Naruto’s rock face.
• So the idea of a [NH] love story wasn’t my idea at the start, however Pierrot staff suggested the movie to revolve around it (hence the final product being the way it is).
• The reason why the movie is set 2 years after the main story is because I wanted [T/N: more like fussed over] the two being in a romance only if they were above 18 (laughs)
Don't know what more you want. This interview literally states he wanted them to be at least 18.