With the first one:
Would you agree that if there are 4 people with ages 7,8,9,10 the average age is 8.5? If you know that recently the average age has increased from 8.5 to let's just say 9 couldn't that mean that either there are less young people (if you take away the 7 year old, the average age is 9) or there are more older people? The answer A states that there are more younger people, so let's say the people become 6,6,6,7,8,9,10. In order to have more younger people, yet have a higher average than before, you'll need to have more older people as well.
6,6,6,7,8,9,10,11,11,11,11. Therefore A supports the conclusion of the stimulus.
I hope that helps...the answer A allows you to eliminate the other option that would increase the average.
With the second one:
The conclusion that there is no reason for ANYONE to make dietary changes for the sake of preventing heart disease is too extreme. In the first sentence, it states that high cholesterol has been associated w/ the development of heart disease and that "many" people with high cholesterol DO NOT develop heart disease. However, SOME people do develop heart disease as due to high cholesterol. Therefore, by stating that NO ONE should make dietary changes would be overlooking the fact that high cholesterol does contribute to heart disease (C).
Hope this helps!