So I hear people saying that Juhabach may have been a shinigami because he fought Yamamoto 1000 years ago and we all know that in the end of the day Quincies are humans. So how can we explain his age?

Well remember the "Schatten Beriech" Hashvald mentioned when Juhabach was about to attack Ichigo?

Maybe the "Schatten Beriech" is a realm where the Quincy Palace is located

The Schatten Beritch could be a variant of the Dangai in that it's separated from time and space. The reason why he couldn't leave for an extended period of time could be that he had to keep himself there in order to access it and I guess that would explain his age.