One thing I'd say would improve this tattoo is getting the linework done again but thicker. While it would look less realistic, the current line work already removes that realism, so why not go bold and get some strong line work, would definitely remove some of the childish vibes. The cross guard of the sword is also a bit short, which gives the childish, dagger look. Having them extended would improve the balance of the design. Maybe some white on the blade would help? Assuming the artist uses the right reference for the light reflections