Nice work! Thanks for posting a solution. Some comments below:
(A) Minor point -- this choice doesn't address "all" old people, just "many," though your point remains -- we have only a claim regarding Sims, not "many" old people.
(C) Essentially, yes. Just because the car's not blue doesn't mean it's red, so to speak. Just because Sims is too old to do the job does NOT imply that "young" people can do the job, and again, "young" has not been defined.
(D)
In effect, the only type of person Rotelle is committed to saying that has the ability to address effectively the issues is someone who is younger than Sims.
Careful! This is essentially correct, but note that Rotelle is definitely not committed to saying that someone younger than Sims can definitely address the issues. For all we know, Rotelle thinks old people are too old and young people are too young and middle-aged people are too in the middle!
What we know, as (D) says, is that the only people Rotelle believes MIGHT be able to do the job must be younger than Sims.