Sarai was 65, but she would live to 127. If the biblical lifespans here are understood literally, she had lived only about half her life. By those standards, she may still have been at an age of full strength and maturity.
Sarai and Abram came from a noble family (see study note on 11:29), so she likely had a regal appearance and wore fine clothing. Pharaoh was drawn to both her beauty and the political advantage of an alliance with her family.