The age of 18 is the border between being or not fully responsible, not only because you are allowed to drive, but also you must take care of what you do. This frontier is not the same as the age of consent, established between 14 and 16 years old, and this difference can provoke controversial situations in some specific cases.
As far as I’m concerned, according to science your body completes all the physical and psychological development at the age of 18, at the age when people start being adults. Even so, in most European countries the age of consent is between two and four years earlier so, why are people prepared to have sex at the age of 15 but they can’t drive until they’re 18?
Despite that controversy, in order to have a relationship you don’t only need the same responsibilities than for driving, but also while being a teenager you can be more mature than people above 20 years old. Furthermore, for having sex you don’t require being fully developed, you just have to be sure about why are you doing it, and most importantly with whom.
Finally, this is a controversial topic so even if you argue with scientific facts about what is the perfect age to start copulating, not everyone will accept it. Because of that, and taking into account all the other facts, the age of seventeen would be, at least, the best one to establish the age of consent, because you’re allowed to have sex before you are 18 in case you are mature, and besides it is not as distant from the current consent age.