In the book Heinlein is quite specific that the actual qualification
is the willingness to risk your life to defend your civilization,
not just to serve in any capacity.
The quote from the book that sums this up comes in the letter
to Juan gets at boot camp from his H&MP teacher:
The Noblest fate that a man can endure is to place his own
mortal body between his loved home and the war's desolation.
The teacher explains that these are not his own words, (That
makes this a fourth-level of quoting) but that Juan would
of course recognize them. I don't. Do you?
A friend told me years ago that Heinlein was inspired about this
form of government by an essay by Rudyard Kipling called
(as I recall) "The Army of a Dream." I just attempted to
validate this on alta vista and didn't get a hit, but the
friend is a usually reliable source. There are Kipling
quotes in "Starship Troopers."