At Least 135 years old
4 Then he left the land of the Chaldeans and settled in Haran. From there, after his father died, God had him move to this country in which you are now living.
NASB Acts 7:4
26 Terah lived seventy years, and became the father of Abram, Nahor and Haran.
NASB Genesis 11:26
32 The days of Terah were two hundred and five years; and Terah died in Haran.
NASB Genesis 11:32
At least 75 years old
4 So Abram went forth as the LORD had spoken to him; and Lot went with him. Now Abram was seventy-five years old when he departed from Haran.
NASB Genesis 12:4
SAB formulation
Acts 7:4 says that Abram didn't leave Haran until after his father died.
Genesis 11:26 says that Abram's father was 70 years old when Abram was born, and Abram's father lived to be 205 (11:32). Clearly, then, Abram was at least 135 when he left Haran.
Yet Genesis 12:4 says he left Haran when he was only 75.
Dilemma
Was Abram 135 years old when he left Haran according to Acts 7:4 or was he 75 years old according to Genesis 12:4? If the last is correct, than Terah has lived still for 60 years while Abram was in the promised land.
Solution
The assumption of the critic is that Terah got three sons in his seventieth year: Abram, Nahor and Haran (Genesis 11:26). However that is not convicing. The text says word-by-word: And Terah was 70 years, and he became the father of Abram, Nahor and Haran. It is more convincing that after his seventieth year these three sons had been born, not in the same year. That means that Abram was born when Terah was 130 years (205 – 75 = 130; Genesis 11:32 combined with 12:4).
The fact that Abram is mentioned first (11:26) while probably he was the last son of Terah, may be due to the important role Abram had in the family. It was a subtle tool of writers to break the order of age to put first a name (or names) of a younger but important one for certain reasons (compare 1 Chronicles 3:5).
See the article: Was Solomon David’s second or fourth son by Bathsheba?
Conclusion
According to Genesis and Acts Abram left Haran at the age of 75. There is no Bible Contradiction in the Holy Texts.
No Bible Contradiction
(update April 2015)