Scott Lemieux explains:

Supreme Court justices tend to come from the federal appellate courts. Particularly since the Reagan administration, Republican presidents have been very conscious of trying to put young conservatives on the federal appellate bench. This is how you get John Roberts and Sam Alito appointed to the Court at a young age, in addition to federal courts stacked with your appointments for a long time. Obama, however, has yet to get a single appellate court judge under 45 appointed to the appellate courts. Since the inauguration of Reagan, "Republicans have appointed 41 federal appellate judges under age 45 to the Democrats’ 10."