Complex sentence: Feeling that this might well be Reagan's last opportunity to pick a Supreme Court justice, the President and his advisers chose Scalia over Bork as Scalia was ten years younger, and would likely serve longer on the Court.
Split into simpler sentences: Feeling that this might well be Reagan's last opportunity to pick a Supreme Court justice, the President and his advisers ultimately chose Scalia over Bork based in part on age. Because Scalia was ten years younger, he would likely serve longer on the Court.