however, you have to remember that people don't just suddenly gain common sense at 18, nor is it completely absent before 18.

I think that is a poor argument. You don't suddenly gain common sense at 16 either, so why not 15? And you don't suddenly gain common sense at 15 as well, so why not 14? You can keep going with this. I draw the line at 18 because a 16-year-old child is a legal dependent of his parents. This means if you have sex and mess up, your parents, whom I presume was not asked for permission to have sex, are now legally on the hook to suffer the consequences of your decision. That means they are putting other people's futures on the line, which I think is the definition of immorality.