Notice that on a first read Terah was aged 70 when his wife birthed Abram (later called Abraham) in Ur of Chaldees and Terah lived to 205 years old before he died in Haran. That would mean that when Terah died Abraham was already aged 135 and had left his father 60 years previously. Notice that Terah had already left Ur of Chaldees to travel to Canaan, but that he had stopped part way in Haran and settled there (Genesis 11:31). We need to understand that God had first called Abram whilst he was still in Ur of Chaldees, and he had only part obeyed God’s call. His father, Terah, left Ur of Chaldees and had taken with him Abram and Abram’s wife Sarah, together with Abraham’s nephew Lot. This would explain, why, when Terah died, Abram now decided to leave and go to a place (in Canaan) that God would show him. It also explains why Abram also felt that he should take Lot with him, since he didn’t feel right leaving him behind when he had been part of the first leg of the journey.
Now as to Abram’s age we know he was 75 when he left Haran and that he did so after his father, Terah had died (Acts 7:2). Therefore, Abram was not the eldest son of Terah, but was 60 years younger than the elder, since he was only aged 75 when he left, and not 135 as we would otherwise have calculated. He is listed first amongst sons in the Biblical record because of his great prominence in the family. So, Abram had been 15 when the Lord originally spoke to him! 60 years later Abram obeyed fully, arrived and the Lord confirmed that He would give this land to Abram. He began to build altars and worship the Lord there.
Abram went to Egypt to survive a drought and Abram foresaw that his wife would prove attractive to the powerful men of Egypt and so he persuaded her to say that she was his sister (she was in fact his half-sister, Genesis 20:12). He saw well, as Pharaoh soon took Sarai to be his wife. Consequently, Abram was treated very well by Pharaoh. The Lord inflicted the Egyptians with diseases until they realised why and sent Abram on his way with his wife and the goods, cattle and servants he had accumulated.
Dear Lord, help me to obey you fully and without delay. In that way I will reap blessings sooner rather than later!