There is no way to filter out certain age groups, to keep them from posting on your questions/opinions. You can look at age and determine which questions you'd like to answer, but can't block them from answering yours unless you block the members individually.
As the asker points out, this is a good feature. There are also people who don't want to do underage questions either (like maybe they're not looking and its some sex question requesting way too much detail).
Being able to filter each side of 18 would be a really good feature.