That's odd. You have to get through FLETC and serve as a Marshal yet there's absolutely no mention of a maximum age.
I know some positions (normally civil investigators) go through FLETC yet have no physical fitness or age requirements. Background investigators, for example, in some agencies can be disabled and still do the job; when they first started training at FLETC, there were some awkward moments for wheelchair users.
Still, this announcement is very different. A solicitation for an executive to run a big operation and, oh-by-the-way, serve when needed as perhaps the only gun-carrying federal LEO on the continent makes this one of the weirdest job announcements I've ever seen.
Serves as a Special Deputy U.S. Marshal. The U.S. Deputy Attorney General has authorized the U.S. Marshal Service to appoint the NSF Station Manager as a Special Deputy U.S. Marshal. As Special Deputy U.S. Marshal the NSF Station Manager is authorized to search, detain and arrest for specific offenses alleged to have been committed by a United States person at any Antarctic location.