They're 18 and 19, they can vote, join the military and go off to war, but they have a curfew??
I'm with Ike. A curfew of any kind doesn't seem reasonable to me at 18+ years of age.
Sure, you can make up whatever rules you want but I hear you asking if you're being reasonable and a curfew doesn't seem reasonable to me. In fact, I think it's awesome they are honoring it at all. I rebelled heavily at that age and would have ignored it and paid whatever consequences were required.