See, what you want is kind of backwards, a device powered by a battery shouldn't turn on a led when battery voltage is too low, because the additional power consumption of the led will drain the battery even faster. You may find some integrated chips that work with two leds, for example to keep a led green above a certain voltage and turn it off and turn on a red led below a voltage but i can't think of any from memory.
It's a bit overkill, but a small microcontroller would do just fine. For example, a
PIC12LF1571 is only 8 pin soic, works from 1.8v to 3.6v so you could power it from 12v using a 4:1 voltage divider (using two resistors) since it uses less than 0.1mA of current at less than 1Mhz (don't need more to light up a led), so the voltage will be 12/4 = 3v when powered from 12v, or 10/4 = 2.5v and will keep running even if voltage drops below 8v.
Basically, all you do is feed the voltage through another voltage divider to an input pin and then the code in your microcontroller would be:
initialize internal oscillator, initialize ADC
for as long as the mcu runs
start to measure the input voltage on the ADC , wait a second or so until the process is done
if the voltage is below a threshold, turn on the led, otherwise turn off the led
wait some time
repeat
Another advantage of using a programmable mcu would be that you could even make the led blink if the voltage is.. let's say below 12v but above 11v.
The downside to microcontrollers is that you need a programmer for them, and a basic knowledge of C (or follow tutorials or ask on forums, the code is so short it can be written in around 6-10 lines on the forum).
Genuine Microchip PIC programmers are kind of expensive (around $40-60) but you can find clones on eBay for much less, for example
here's one for <$15
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