Could you explain this a little more? I don't quite see your reasoning. Leaving aside the fact that "morally valuable" seems too vague to me to be meaningfully measured anyway, adults aren't immutably fixed at a "moral level" at any given age. Andrei "Rostov Ripper" Chikatilo didn't take up murdering people until he was in his forties. At twenty, he hadn't proven anything.

Bob at twenty years old hasn't murdered anybody, though Bob at forty might. Now you can say that we have more data about Bob at twenty than we do about Bob at ten, and therefore are able to make more accurate predictions based on his track record, but by that logic Bob is at his most morally valuable when he's gasping his last on a hospital bed at 83, because we can be almost certain at that point that he's not going to do anything apart from shuffle off the mortal coil.

And if "more or less likely to commit harmful acts in future" is our metric of moral value, then children who are abused, for example, are less morally valuable than children who aren't, because they're more likely to commit crimes. That's not intended to put any words in your mouth by the way, I'm just saying that when I try to follow your reasoning it leads me to weird places. I'd be interested to see you explain your position in more detail.