The first thing that must be addressed for this issue to be properly considered is exactly what is meant by the phrase "age of consent". Are we to strictly consider it for the purposes of this discussion completely separate from all of the other issues where the State has a compelling interest in the various "age of consent" or "age of majority" laws? If so, it will merely serve as an entertaining academic discussion, that has no real meaning outside the strict context of this thread. If we are, on the other hand, to properly consider the "age of consent" as it applies to all other aspects of our society, then we must consider it in the context of the age at which a young person may drive, vote, enlist in the military, enter into a binding contract, purchase alcohol or a firearm.
I reject as a waste of time any notion that it is intellectually honest, or even prudent to engage in the former, so I shall address the latter.
While there is no argument that sexual relations between consenting adults is of no concern of the governments, except under the most narrow exceptions, the fact remains that the State does have a compelling interest when that conduct is among those who are of an age, adjudged by the State, to be mentally inadequate to be able to provide informed consent. The same principles apply when dealing with those who are of the age of majority, yet are mentally disabled, and therefore not able to make a proper "informed consent" to engage in sexual relations.
The same standard applies to all aspects of life, as the State, having a compelling interest in securing the blessings of liberty to it's citizens, must also ensure that it denies to those who are unable, through disability of mental capability, from engaging in any activity that could render them responsible for damages, debt, or disrepute before they have the ability to fully comprehend the responsibilities they are undertaking.
If we are to assume that there should be no "age of consent" for engaging in sexual relations, from which another life may be formed, are we then to abolish the "age of consent" for which one may purchase a firearm? For which one may operate a motor vehicle? For which one may purchase and consume alcohol and tobacco? For which they may enter into contracts? For which one may enlist in the military service and therefore be sent into combat? NO, of course not, therefore there is no logical reason to believe that doing away with the "age of consent" for sexual relations would be any more advantageous to society.
Even when we refer to the father of American jurisprudence, Lord William Blackstone, he clearly defines the age of majority at 21 years of age, yet he also acknowledges throughout his commentaries that those younger than 21 years of age may engage in other activities, including marriage, at the age of 12 for girls and 14 for boys, WITH their parents consent. The fact is that the various "ages of consent" are intentionally graduated, dependent upon the degree of responsibility attendant with the right or privilege being bestowed, congruent with the average mental ability of the person to honor the responsibilities that accompany those rights and privileges.