Hey, everybody! I'm a bit late to the discussion, but I thought I'd try and shed some light on how much time has passed between the end of the original series and the start of the sequel.
It's never explicitly stated, but Gash went from 6 to 19... because 13 years have passed. Kiyomaro assumed he was 14 years old by his APPEARANCE alone, because the sad truth is that it seems Kiyomaro never bothered to learn how old Gash actually is during their time together. The whole point of Kiyomaro saying that he looked 14 was to play into a gag where Gash felt self-conscious about his growth development, so Gash turned his nose up and tried to save face by claiming all the demons age differently... when we now know that, based on how all the other demons have been aging, Gash is the only one who is a late bloomer. He just doesn't want to admit it.
Gash, Kanchome, and Tio are all 19 years old.
Kiyomaro is 27 years old.
Apollo is 39 years old.
Folgore is 36 years old.
Megumi is 29 years old.
Barry is 30 years old.
Sherry is 31 years old.
So far, we haven't seen anything that contradicts this. While it may be possible that time might move at a different rate between the human world and demon world, I don't think we're supposed to be thinking about it that hard. The bottom line is that, if we're to believe 13 years have passed, then everyone except for Gash seems to corroborate that based on their physical appearances after the time skip.