“American Idol” hit a series low on Thursday night, dropping to a 2.7 in the key demo and 11.03 million viewers. If 10 million is the benchmark for disaster, then it’s just around the corner. At this rate, the moment could come earlier than May 8th, when “The Voice” will put a show up against an episode of “Idol.” Of course “Idol” won the night last night anyway, beating a lot of weak shows and reruns. Advertisers will look at the sheer numbers and see that of all people watching TV last night, most of them were seeing “Idol.” But still– every week “Idol” sinks lower and lower. And in the key demo, “The Voice” is killing it already and it’s only been back for one week. That 2.7 means “Idol” is skewing older and older– not the audience that buys and downloads music. It’s more or less tying with “Grey’s Anatomy” (which should have been cancelled eons ago) and Kerry Washington’s cool “Scandal.” Now let’s see how “The Voice” does this coming Monday.