There is value to knowing who your consumers are, but the typical "demographics" research typically falls short of drawing important correlation to who consumers are, and what motivates them to buy. Does being 32 lead someone to purchase Crest toothpaste versus Colgate? Moreover, some demographic categorizations do not even make sense. 18-49 is often used to describe a consumer group. Aside from "people who most likely have jobs" this age range has very little in common simply because they are within an age group. Of course you will find 18 year olds who buy the same products at the same store of 49 year olds. How does that help you better position your product?
There are times when an age group is an important factor to consider. The important thing to remember is to question those demographics rather than jumping to a conclusion and alienating your customers because someone showed you a statistic.