couldn't he just pick that up over time?
people don't speak the same way all their life.
Well, though I haven't spoken with a lot of old Japanese people when I was in Japan (and when I did, oh boy was it hard to understand them), it's always been my understanding, though I may be wrong, that stuff like "ja" instead of "da" or "washi" instead of "boku", "ore" or "watashi" are not indicative of age/growing old so much as old-fashioned/archaic ways of speaking. i.e. they're the way people would be speaking back in the day when the character was young. So if anything, Joseph should be using "ja" a lot back in Battle Tendency because Battle Tendency happens before WW2, and either stick with it or use it less as his speech pattern evolves with the way language itself evolves. But then, that means everyone should speak like that in Battle Tendency, and I guess that would sound weird too.
Your argument is a good one, but it only applies to some of his trademark expressions in Stardust like "Oh no!" and "Oh my God!". For some reason he started using those a lot as he went through life, because, as you said, people don't speak the same way throughout their lives.
[EDIT] For the record I doubt Japanese people were all like "ja" and "washi" pre-WW2. It's probably older than that, and possibly a rural thing. But if people were speaking the way they actually spoke back in the day, nobody would understand or want to watch the show. It would just sound too weird.
. It wouldn't be weird if Joseph was always known as an old dude from the get-go, as that's the way old people speak in most anime, but it's just weird when his younger self didn't talk like that at all. Anime trope gonna trope I guess.
