Answer:
Give it food and it will live, give it water and it will die.
Different lights do make me strange, thus into different sizes I will change.
Look into my face and I'm everybody. Scratch my back and I'm nobody.
It's been around for millions of years, but is never more than a month old. What is it?
Sleeping during the day, I hide away. Watchful through the night, I open at dawn's light. But only for the briefest time, do I shine. And then I hide away. And sleep through the day.
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.