Dude, not only is 18 over the "age of consent"
(as if that has anything to do with this), 18 is also over the age of majority; the guy who displayed the sign was officially an adult, able to vote, and able to get shot all to hell in a war manufactured by the guy he most likely didn't vote for.
jb4
"It's hard for me, you know, living in this beautiful White House, to give you a firsthand assessment."
— George W. Bush, when asked if he believed Iraq was in a state of civil war (Newsweek, 26 Feb 07)