Dr Kim came up with the poom belt, taking the idea from Korean civil service. They used the half red & half black belt. But why those colors? What do they symbolize & what does the combo symbolize?
I would say (from my position) that as red belt is the last full colour belt, then red belt-black tags is the last kup grade that a half-half belt would show that it's more than the last kup grade but not quite a full dan grade. That seems logical and strikes a chord with how I see the poom grades.
As for what red symbolizes, I'd been told many years ago that the colours symbolize progression outwards from the centre of the earth:
White - the white hot fiery centre of the earth's molten core
Yellow - the cooling, but still hot central crust
Green - the grass on the surface of the earth
Blue - the sky
Red - the sun
Black - the wider universe/space
That also fits with the colour belt -> next colour tag idea - you're getting closer to that next stage so you get a little bit of the next colour in your layer before you move in to that layer.
Would anyone want to really learn a martial art from a teenage master or a GM in their 20s? Or even 30s?
I'd say that there are two questions - would anyone want to learn from an instructor in their 20s or 30s? In this case I'd say obviously yes - some of the world's best coaches are in their 30s (or were when they hit their prime). I personally taught a small children's class as an 18 year old 2nd Dan (with hindsight I wasn't mentally mature enough to understand the business side of running the club).
Whether they are a Master or Grandmaster hasn't particularly mattered in my area, a teacher is a teacher (I'm learning BJJ from a purple belt and that's fine to me - he knows more than me and is allowed to teach at purple belt so it's cool).
Whether they should be a master or GM in their 20s or even 30s is a different question, but from a student's point of view I don't imagine most would care and would trust the certifying authority that gave them the grade.
What about the maturity that only comes with age & experience?
I agree with you that you shouldn't be a GM in your 20s/30s - I was just saying that a lot of people would be happy learning from them.