While persons under the age of 18 may be considered "minors" in some parts of the world, in other places, they are not; in Japan, for instance, the age of consent is 13 in some prefectures.
Really though, the simple biological fact is that by the age of 12, the average female in the United States is already 2 years into puberty and capable of reproduction, with males not far behind. In other words, by the time the average girl turns 18, she's already been having her period for almost 6 years, and were she in a sexual relationship with a fertile partner for this time, she could have already given birth to 7 children.
I am not saying that a 12-year-old is that responsible. Nor I am saying that people cannot be manipulated into sexual behaviour; a relative of mine (who is a lesbian) knows of several self-proclaimed homosexuals that regularly engage in sexual activities with members of the opposite sex, simply so that they can have fun without having to deal with the consequences of a long-term relationship.
But we must be realistic. It is not God who is saying "don't have sex until you are 18"; it is the law, and our so-called 'democratic' capitalist society, which has all but ensured that anyone who wants to be able to provide for their family must spend 5 (if not 10) years of their post-pusbescent lives in full-time studies, just to put food on the table. Any way that we as Christians can help homosexuals (and heterosexuals alike) deal with such a turbulent time in their lives should be fully promoted. After all, if we aren't going to help them, who will?
Peace,
Nick