Kliu,
This is sufficient-necessary reasoning and I think it is most effectively attacked using SN diagramming. The heart of The argument is this:
If Sims age (or older), then cannot address difficult issues. (CP: If can address difficult issues, then younger than Sims)
Sims is Sims age (obviously).
Therefore, Sims cannot address difficult issues.
S
SA
CADI
S
CADI
B can be diagrammed as
CADI SA. This is also known as a double not not arrow and is very confusing. The contrapositive would be If younger than Sims, then can address difficult issues. The negation would be: If one cannot address difficult issues then you are younger than Sims. Or to contraposit that- If Sims age or older, then one can address difficult issues. So, this does weaken the argument but doesn't destroy it, because the conclusion is that Sims cannot address difficult issues. Even if he is capable by age, perhaps there are other reasons he cannot.
As for D, it diagrams to CADI
SA so the negation is (we simply negate the necessary condition) CADI
SA or in plain English: If one is capable of addressing difficult issues, one must be Sims age or older. We see this is a direct contradiction to the argument and, if accepted, would make it invalid.
Hope that makes sense. The basic idea is I wouldn't use Assumption Negation Technique but if you do, to negate a sufficient-necessary statement, you logically negate the necessary condition only.