Positions in the federal magistracy are important legal posts that carry significant responsibilities and authority. Given that United States judges are appointed to serve for life, the criteria for their selection must set at a high bar.
First, a federal judge must be a qualified attorney. While this is not a constitutional requirement, judges without such qualification are unlikely to win Senate confirmation. For instance, it's been more than a half-century since a justice has been appointed to the United States Supreme Court who did not have a law degree (the last was Robert H. Jackson, appointed in 1941).
Second, a judge should be of an age that they will qualify for senior status under the "Rule of 80." The Rule of 80 establishes that a judge may assume senior status at age 65 provided they have at least 15 years federal judicial experience. Senior status allows older judges to go into voluntary semi-retirement which is generally considered beneficial in that it discourages elderly and infirm justices from continued service.
A third consideration is the candidate's prior courtroom experience. For instance, more than 46 percent of recently appointed U.S. District Court judges were serving as state judges or federal magistrates prior to their appointment. The bulk of the rest were attorneys with active litigation careers, many of whom had previous judicial experience.
Other factors in filling federal judicial posts include a candidate's honesty, integrity, and overall legal acumen.