I see it over and over again. The idea that the age of consent must be there for a reason. That whoever put those laws in place must have known what they were doing.
The law is not a divine commandment. The people who write laws are not unknowable wise men with inscrutable wisdom far divorced from the ability of the common man to understand. The entire point of studying history is so that we can follow the progression, the whys and wherefores that lead us to the world we live in today.
The age of consent came into being in the days of women as property. Marriage was a business transaction between the groom and the bride's father, with the groom paying a bride-price to her father. A virgin bride commanded a high price, and thus anyone who had sex with a man's daughter was literally diminishing the value of his property. The age of consent was a way of protecting the investment, not protecting the person.
Come the turn of the 20th century, a group of religious conservatives, terrified of the idea of women in the workplace, lobbied hard for the age of consent to be raised to its current level. Their writings are still around explaining their concerns. To summarize, they were afraid that women working outside the home might meet men and want to have sex without getting married. They used the age of consent to push their social agenda. That's why to this day there are still states which have exemptions in their ages of consent if the parents consent, or if the couple are married.
After that, the age of consent was fiddled around with to discriminate against homosexuals. Ages of consent were made different for different depending on the genders of the partners and the acts performed. It was a way of hurting gays where legislation couldn't be passed to ban such activities entirely. The famous Stonewall riots were partially a response to this kind of discrimination.
The age of consent has never been the well-reasoned compromise between the twin noble ideals of individual sexual liberty and the protection of the vulnerable that its supporters claim it to be. The age of consent is a historical accident built on a foundation of social mores our culture rejected long ago, and discrimination that even the bigots of our day at least pretend to be ashamed of.
It's long past time we wrote a law that reflects what informed consent actually means. Long past time we actually had the reasoned discourse that everyone who blindly accepts the age of consent as it stands assumed happened at some unspecified point in the past.