Good point; his youth is often overlooked. It seems like Mills has been around forever but he's just 24 years old. Point guards usually don't start figuring it all out until they are around 28, so he has plenty of time to improve. It's still difficult to pinpoint his ceiling -- there's a chance he could become a full-time starter one day ... there's also a chance that he flunks out as a PG and will have to switch to SG.
I've liked what I've seen out of Mills in these past two games. AFIAK, these were the first two games all season that Pop allowed Mills to be the backup point guard behind Parker in both halves. Mills hasn't been great in these last two but he also hasn't hurt the team. He's making strides with his passing, which is really important.
Can Mills become the backup point guard on this Spurs team? I'm not sure yet. In a perfect world, you'd have a lockdown defender who is also an ace ballhandler next to Ginobili coming off the bench. Mills isn't that but he has strengths (speed, shooting, energy, etc.) that could help the second unit.
Truthfully, there aren't many better situations for him. In fact, this might be the best situation for Mills. He's not a playmaker, so being able to allow Ginobili to make the plays while he basically gets to play shooting guard in the halfcourt offense fits him perfectly. When Ginobili is healthy, Mills usually isn't asked to run plays ... his mission is to dribble the ball up the court successfully and then either attack or give it to Ginobili.
CJ makes some sense because he best fits the most desired mold (ballhanlder who can defend). But he's also very green and it's probably too late for him to jump over NDC, Neal and Mills to land the gig. I'd like for Pop to give him a shot since Pop is in mad scientist mode anyways ... but I think it's a long-shot that CJ ends up being the answer.