Yes, have to join the Navy or Marines for an extended time. Government is not going to waste time training anyone who only signs up for a 3 year enlistment. Do not have to be a citizen of the U.S. but will have a very serious background check (drugs included) like even U.S, citizens do. Got the info from current and past SEAL's. I live close to the west coast S.E.A.L. training site, near San Diego, most of my life.
At 36, most former SEALS are retired and hanging around bars near PCH (they got their own bars). A very few may be going for 20 years service, winding up working staff or such. Not a good idea to be married while in service, never seems to work well. Duty first above all, marriage a distance second. Some good job opportunity's after leaving service, with lot's of travel time included.
Main requirement to becoming part of the SEAL force is a solid mental attitude and confidence to go along with that. Second is physical abilities. Focus and stamina (both mental & physical) have to balance each other out.