You must not understand the concept of a crossover and fanon. And she’s a teenager in the English dub.
Just because something is a crossover/fanon doesn’t mean people aren’t going to notice the age gap. Though the fact the artist calls her “Bulla” does suggests she’s meant to be seen as a teen in this pic.
This was made with the intent and image that Bulla and Bakugo and Bakugo are the same age despite canon. Additionally, she’s a teenager going by the English dub. So no, she’s not six years younger.
If this is contradicted by information in the original language text, then it's a bad dub.
I think Toonami and Funimation just didn’t want to have to explain to American parents why their kids are watching 10-year-old girls running around in crop tops (despite not having any issues with young Chichi running around in bikini armor in Dragon Ball).
He's actually older than that, Bakugo was 16 for a lot of the series, and according to the original dub of GT, GT is oly 5 years after End of DBZ, and Pan was confirmed to be 4 at the EoZ, so she's 9 during GT, and Bra/Bulla is a year or so younger Pan, making her 8 to possibly 9 years old, making Katsuki between 7 to 8 years older than her
