Brenda Ann wrote:

[National AM] is where you sell national products. People buy Coke,
Pepsi, STP, Quaker State (and Quaker Oats) everywhere. Most
nighttime radio has long been such spots (as has network radio).


Advertisers are not interested in anybody older than 35. Their
proclaimed reason: People over 35 are "set in their ways" and no
amount of advertising is going to make them switch brands. For
example, if you've used Crest for the last twenty years, no number of
ads is going to make you switch to Colgate.

BUT:

The young teens and adults are "undecided". They have no brand
loyalty, and those are the people advertisers want to target. 35 and
under.

Thus national AM with its over 45 crowd is extremely UNattractive to
advertisers.