Pesonally, I don't think premarital sex is inherently wrong. I suppose it can be "wrong" if practised irresponsibly.
I also believe that those who claim sex outside of marriage to be wrong are merely creating taboos that entice people--young people especially--to break them. I could be wrong, but it seems to me the worst thing a parent or other authority figure can do is to arbitrarily tell a teenager who is raging with hormones and a natural desire to be independent NOT to engage in any sexual activity because it is wrong and impermissable. Whatever.
In my humble opinion, homosexuality is neither right nor wrong. It simply is. It may sound like a cop out, but that is a personal decision everyone must make for themselves. It is nobody else's concern. And while many religions may approach the subject differently, I personally believe if there is a God, only He/She is infallible and therefore the only acceptible Judge of our deeds. So, in that regard, nobody holds the authority or the right to condemn another for sexual preference.
Finally, as to the most appropriate age for sex, that is again solely up to the individual. And I am certain it varies from person to person depending on their emotional and physical maturity. Just because one is physically capable of having sex, certainly doesn't mean they are emotionally mature enough or responsible enough to be doing so.