There are two aspects to the age of consent that make it either ok or outright criminal to set at an age below 21
The first aspect is mental maturity. Japanese people are one of the world peoples with a very high average IQ, and East Asians are generally known for reaching their full mental maturity/capacity rather early (around 16-18 iirc).
This means that even at ages below 15,they will already have on average a higher IQ than the majority of people on this planet, since even White Europeans on average have an IQ of 85+ at the age 11-14, which is as high as West&South Asian adults have on average, and way higher than Sub-Saharan Africans have (in the low 70s as adults).
So, if you argue about this, and Japanese cannot give consent at this point, the majority of people on earth is condemned to eternal sexual immaturity.
However, there is the second aspect,too, which makes this rather disturbing.
The second aspect is the sexual maturity of the body, since sex is an act of and primarily for reproduction(a fact many people still deny to this day even though science has proven this, and yet people wonder why some believe the earth is flat against scientific evidence).
So, by putting the age of consent at 13, you're stating that it is ok for a 13 year old to have a child, even though early teen pregnancy can endanger the health of a woman.
So,if we combine both mental and bodily features, we can conclude that an age of consent below 16 is rubbish, and in general it should be not below 21 years of age because for the majority of people in industrialized nations it's impossible to become independent before that age and thus unable to care for a child.
There is also the issue for women that the chances to ever have a lasting relationship/marriage with a man decreases with the number of sexual partners she had before, so having a high age of consent actually protects women from destroying their future happiness with irresponsible sexual habits.