As Queen of Aquitaine, I was pressing a weak claim war on the Duchy of Anjou on behalf of one of my vassals when he died, causing the war to be cancelled. The vassal's titles were inherited by his (12-year-old) son; as, of course, was the claim on Anjou because it had been pressed in war. The son is, naturally, still my vassal, and the Claimants page for Anjou shows him at the top of the list (which I assume means he's first in line).
However, I am now not able to Declare War over Anjou again, and the tooltip says it's because I don't have a valid Casus Belli. Why is this? I have a vassal with a weak claim on the Duchy. Is it because he's still under 16?