In my opinion the appropiate age should be 17 or 18
because face it, us kids are stubborn and will usually not listen to adults an our parents most of the times.
There are so many girls losing there virginities at the age of 13, 14, 15, that it's ridicoulous(sorry if i spelled it wrong XD). They are basically, just doing whatever the heck they want, just acting it out without thinking it thru.
So in my opinion where you are 17 or 18 you have had all the time in the world to think it thru and talk about it with your parents, and make the right assured decision, to use a condom.
Lolz i'm only 15 really but 17 for me should be and appropiate age =D