Why has my cat suddenly begun playing with his tail?

He is 10 years old and has never done this until just this month. He lays on the floor and seems pretty distracted until he sees his tail twitch, then he stares at it and moves it real slow then twists and pounces on it then occasionally chews on it and begins the process over. So, he is totally aware it's his own tail, because it's like he uses it like a toy. Is he bored? I'm not worried. I think it's hilarious, just curious.