I have looked and looked and have not been able to find a single Bible mention of a person marrying at the ages of 12-16.
The Bible permits marriage once a girl/woman has reached the age where she has a regular monthly cycle, and she has her father's permission. Remember that while today, with better nutrition, etc., it is not unusual for a girl to begin her cycle at 12, or even younger. But during the time of the Bible, the average was later, usually 15-16 years of age, according to medical and other text that survive from that time.
A child before the age of 15 has a very difficult time carrying a baby to full term and delivering it without endangering their own live. Even though they may be having a period, and thus capable of getting pregnant, they are not yet physically mature enough to be pregnant. Restrictions need to stay in place until a girl is old enough to safely have a pregnancy.
One of the reasons that God does not give an "age of constent" in scripture is because he knows that society will change over the ages. When it was 99% farms and hunting, and families stayed together as tribes, a child at 15-17 could managed a family. All they need was some simple hunting and farming skills. Plus they had the safety net of the tribe/family.
Today, we an education is needed to be able to survive, a child trying to raise a family at 15 would have little chance of success. Children of such parents are usually doomed to a life of poverty. Today, it is not unreasonable to require a man or woman to have reached an age where they at LEAST have a high school education before they marry and try to raise a family.
Why are parents upset? Because they have the wisdom and experience to know how destructive such sexual relationships and marriages are for the teens involved.
The Bible's requirements for the "age of marriage" have not changed. So far we have only discussed the first of the two requirements - a monthly cycle. The second is parental permission. The laws of sexual consent and marriage age are the implimentation of that second requirement.