Premise: average age in the region has increased to 57
Conclusion: in the last 10 years, people 65+ have increased dramatically in the region
(D) cannot be the correct answer because we don't know the ages of those who moved in/out of the region. It could be that a bunch of 57 year old people moved in and everyone else moved out, and therefore the conclusion would not hold.
(A) states that a bunch of people under 18 have moved into the region. Therefore, in order for the average age to rise even though the number of significantly younger people increase, it must be the case that significantly older people, 65+ have also increased in order to pull the average up.