‘So Abram went as Yahweh had spoken to him, and Lot went with him. And Abram was seventy five years old when he left Haran.'
Abram obeyed the voice of God. Lot, his nephew also went with him. It is very probable that through Abram's witness Lot too had begun to worship Yahweh. It is possibly difficult to comprehend what a major step for Abram this move was. To the ancients membership of the tribe was a sacred duty and to leave it was to dismember the tribe. But Abram has the call of Yahweh and his act is therefore a declaration of faith.
Abram's age at leaving (seventy five) indicates according to the Hebrew text that Terah was still alive when he left. Terah was ‘seventy' when he begat Abram (Genesis 11:26). Seventy plus seventy five (Genesis 12:4) is one hundred and forty five. Terah died at two hundred and five. Thus he would live for another sixty years.
However we have already seen that the ‘seventy' indicates a divinely perfect birth and the seventy and five here may suggest the divinely perfect time (seven intensified) plus five (the covenant number). It is explaining why Abram acts at this point in time. Thus the numbers may not be intended as literal numbers. Furthermore the Samaritan Pentateuch gives Terah's age on death as one hundred and forty five. It thus sees Abram as leaving Haran on the death of his father. This is the tradition known to Stephen in Acts 7.
(The Samaritan Pentateuch, comprising the five books of Moses, is a separate and very ancient tradition of the Hebrew text, which, with a few particular alterations, was preserved by the Samaritans).