What is the best age to have sex?

When mature enough: There is no specific best age to have sex, although below adulthood it is fraught with difficulty. When both people are mature enough to be in a loving, respectful, committed relationship and also to understand the responsibilities involved with pregnancy, birth control, and potential std's, it might be feasible.

Not an easy answer: Being ready for a sexual relationship involves more than your physical readiness. Anyone who has had puberty could have sex, but most people are not emotionally/intellectually ready at that age. Readiness for sex involves being responsible, making good choices, protecting yourself, understanding the possible risks, and readiness for a healthy relationship with another person.