How old? Estimates vary, but as a general principle, nitromethane will evaporate even when the fuel is sitting. It is recommended that it be stored in a cool location. Away from sunlight. As fully sealed as you can get the jug. What will basically happen is the nitro content of the fuel will slowly decrease, increasing the relative oil content and methanol content. So basically your engine will run as if it's on a lower nitro fuel. As for how long fuel is considered "good" for, I'll have to dig to find a good estimate.