When the character was first introduced, Giles said that he was "Not yet two hundred years old. In the "Touched" scene, Spike mentions "a hundred and twenty five bloody years," or some such, but it was never clear whether either number included his human age, or just the time he was a vampire. My guess is that he would have been born (not sired) around a hundred and fifty to a hundred and seventy five years ago, or somewhere between 1835 and 1857.