There was a time in human history when adulthood was believed to have started in the early teen years. Life is much more complex these days and it takes longer to educate a child and prepare him/her for adulthood than it did in past centuries.

Very few people have the means to be financially independent without at least graduating from high school, let alone receiving vocational training or a college degree. Children suffer a great deal when their parents are inexperienced, immature, and financially struggling.

Scientific research has shown that the brains of teenagers aren't even fully formed by the time they achieve legal majority. Growth and maturation continue well into the 20s.

My mother waited until her mid-20s before having her first child. By that time, she had a husband, a college degree, and a stable income. The birth of her first child was still a tremendous shock and marked a huge change in her life. You can't tell me an experience that a 25 year old woman with a supportive family finds overwhelming is a good idea for a 15 year old to experience.