(Joshua 24:15) Now if it is bad in YOUR eyes to serve Jehovah, choose for yourselves today whom YOU will serve, whether the gods that YOUR forefathers who were on the other side of the River served or the gods of the Am′or·ites in whose land YOU are dwelling. But as for me and my household, we shall serve Jehovah.”
(Isaiah 42:8) “I am Jehovah. That is my name; and to no one else shall I give my own glory, neither my praise to graven images.
(Isaiah 45:5) I am Jehovah, and there is no one else. With the exception of me there is no God. I shall closely gird you, although you have not known me,
I think the last verse speaks for itself
As for how old the Jehovah's Witness's are ...
According to the Bible, the line of witnesses of Jehovah reaches back to faithful Abel. Hebrews 11:4–12:1 says: “By faith Abel offered God a sacrifice of greater worth than Cain . . . By faith Noah, after being given divine warning of things not yet beheld, showed godly fear . . . By faith Abraham, when he was called, obeyed in going out into a place he was destined to receive as an inheritance . . . By faith Moses, when grown up, refused to be called the son of the daughter of Pharaoh, choosing to be ill-treated with the people of God rather than to have the temporary enjoyment of sin . . . So, then, because we have so great a cloud of witnesses surrounding us, let us also put off every weight and the sin that easily entangles us, and let us run with endurance the race that is set before us.”
With reference to Jesus Christ, the Bible states: “These are the things that the Amen says, the faithful and true witness, the beginning of the creation by God.” Of whom was he a witness? He himself said that he made his Father’s name manifest. He was the foremost witness of Jehovah.—Rev. 3:14; John 17:6