6+ is not important, nor are monthly books. Business is done on 25-54 adults, 25-54 women, 25-54 men and then 18-34 on a three book average, some advertisers look at a six book.
Actually buys can be much more specific, but they almost always fall in the broad 18-49 spectrum.
There are English dominant Hispanic Females between 25 and 44 buys. There are male 25-44 buys. There are broad 25-54 buys but they are made with some stations strong in the lower end, some in the middle and some at the top. There are income-based buys. There are even buys that look for strength in certain metro counties where the advertiser has locations. There are "no talk stations" buys, and there are "sports only" buys.
But your point is accurate: under 18 and over 55 are not demos that get much, if any buying activity.
And it can't be said enough that month to month wobbles are not significant unless they are multiple month trends and agencies seldom buy off one book... and they always discard the December and Holiday books in averages.