The team for an Olympic soccer team is picked by the national team coach. However, for the Olympic soccer torunament, there are some rules as to who can and can not play, for example age restrictions.
To be on a professional soccer team you have to be at least 18 years old.
Olympic games soccer is not won by a person, but by a national team. In 2004 it was Argentina.
The same way all other sports do: by qualification.
you must play soccer
You can be as young as 15 to join a pro soccer team