Joke:

Q: What do you call someone who graduates last in their medical school class?
A. "Doctor".

I think it's funny. Setting aside the humor, the insinuation is that a lot of doctors might be incompetent. But the answer that is consistent with the idea of doctor competence is that if the medical school class had been twice as big, many people who graduated with even lower test scores would also be good, competent doctors. Incompetence would gradually creep in as scores got lower. By keeping a high bar for medical school admissions and for a passing grade, we make sure the ones who pass are competent, even if most with somewhat lower scores would be too. At least that's a theory of how it might work, regardless of how it works in practice.

One common objection to the age of consent is that it is an arbitrary line. Suppose the age of consent is 16. How can sex be terrible on the evening before someone's 16th birthday but perfectly OK after midnight?

I would think thoughtful people who support the age of consent would say that it does not become perfectly OK after midnight. They still might think it is unwise and often exploitative, but just not quite so risky that they want to make it illegal. Clearly many people still disapprove strongly of 19-year-olds involved with 45-year-olds. It's reasonable to think that the potential for exploitation in such relationships is higher than if the younger party was 25.

On the other hand, it's a good bet that many prosecutors would understand that the willing 15-year-old who has sex really is pretty mature and decide not to press charges. It's a fair bet that quite a few parents know their 13- or 14-year-old daughters are having sex with adult men but don't intervene. Whether you think their judgment is right or wrong, it recognizes the continuum of age. The average person's alarm would be much greater if the daughter was 9 or 10.

The law sets a single age of consent in the absence of the great bureaucratic overhead of competency tests. It does the same with a voting age and an age of legal emancipation. You can sign binding contracts when you turn 18, but people would advise you to get lots of advice from older people before you do. Similarly, if you are under 18, your guardians would do well to consider your own opinions very seriously even if they make the final decision.

This of course only addresses one aspect of the age of consent, not what that arbitrary number should be or whether there should be one at all.