The duty of the interpretation of the law rests in the Judiciary. The highest court in the United States, above all others, with the final say in the what laws are Constitutional and which aren't, is the Supreme Court. There is actually very little said in the Constitution about the Supreme Court or any of the courts. Article 3 is the shortest of the first three articles, and only the first two of the three sections have anything to do with the structure of the Judiciary. The Chief Justice is only mentioned in Article 2, concerning presidential impeachment.
Judges have no Constitutionally mandated age, residency, or citizenship requirements.
Judges can be impeached by the Legislative branch. Currently, there are nine justices of the Supreme Court. Since the Constitution does not specify the number, it has fluctuated, from as few as five to as many as ten. The Supreme Court is the highest appellate court, meaning that cases normally only come to the Court by way of appeal after appeal of the losing party. The Supreme Court does have original jurisdiction of a few types of cases, spelled out in Section 2 .