Yes, "Demon Slayer: Kimetsu no Yaiba the Movie: Mugen Train" is the same film. It is an adaptation of the "Mugen Train" arc from the "Demon Slayer: Kimetsu no Yaiba" anime series. Released in 2020, it follows Tanjiro and his friends as they face demons aboard the Mugen Train. The movie became a significant success, breaking box office records and receiving critical acclaim for its animation and storytelling.

"Demon Slayer: Kimetsu no Yaiba the Movie: Mugen Train" is a major hit. Released in 2020, it became the highest-grossing film in Japan's box office history, surpassing even Studio Ghibli's "Spirited Away." The film grossed over $500 million worldwide, receiving critical acclaim for its animation, story, and character development. Its success significantly boosted the popularity of the "Demon Slayer" franchise, making it a cultural phenomenon. Overall, it is widely regarded as a triumph in the anime film industry.

In "Demon Slayer: Kimetsu no Yaiba the Movie: Mugen Train," Tanjiro Kamado is 16 years old. The film continues the story from the anime series and follows Tanjiro and his friends as they embark on a mission aboard the Mugen Train to investigate a series of mysterious disappearances connected to a powerful demon. Tanjiro's age is consistent with his character development throughout the series, as he grows and faces new challenges in his quest to save his sister and defeat demons.

"Demon Slayer: Kimetsu no Yaiba the Movie: Mugen Train" is not the same as the "Infinity Castle" arc, which is part of the ongoing "Demon Slayer" anime series. To watch "Mugen Train," you can find it on various streaming platforms like Funimation, Crunchyroll, and Netflix, depending on your region. For the "Infinity Castle" arc, you should follow the latest episodes of the "Demon Slayer" anime, which continues the story after the "Mugen Train" movie. Always check your local streaming services for availability.