Young Athletes Agility
Personally, I have never thought of flexibility or mobility as a factor that plays a huge role in an athlete being able to assume an athletic stance. I do not see where there is enough flexion/extension in any joint throughout the chain where this is really an issue. If you are getting that low you are never going to be overly quick out of the position.
For me, it has always been a matter of reeducation. Young athletes simply have no idea how to align their bodies to create the most effective angles for spontaneous multi directional movement. Often they have been coached wrong or not coached at all and have created their own interpretation of the stance.
So then it comes down to teaching. Therefore, one must be careful with their “selection of words” when describing movements and positions to kids and young.
For instance, flat back can often also mean a completely vertical torso. MANY kids will automatically make this correlation (and so do many coaches.)
I prefer using the terms “neutral” and “tilted.” As Kwame suggested, we work on rounding the spine, we work on arching the spine, and we work on keeping the spine in a “neutral” alignment. Then it is a matter or “tilting” the neutral spine forward as the hips move back.