Both Voyager 1 and 2 use RTG's (Radioisotope Thermoelectric Generators)[1] for power. Every year the RTG puts out less and less power, so instruments are gradually powered down to preserve the functionality of the critical ones for as long as possible.
According to JPL[2] at least one scientific instrument will continue functioning until about 2025, and the crafts will remain in range of the Deep Space Network until 2036.
Let's just hope the Deep Space Network will continue being funded. Further capsizing its budget and shutting it down would make it impossible to stay in contact with one of the greatest achievement of Mankind.
If you ever have the chance to check out JPL during one of their open houses, I highly recommend it. They have an exhibit with replica models of various satellites throughout history, and in the entry way there are screens showing data about what is currently being pulled in from the DSN. When I was there, it was pulling down data from New Horizons (at some absurdly low bit rate)!