Hey there! I'm happy to help!
Being sexually mature means that your body produces sex hormones that allow a baby to fertilized in a female's uterus and develop and be born. When teens hit puberty, many of these hormones begin to develop, so they technically could have a baby and be a parent. However, at an age like 12, you are still going through school and growing up yourself. People that age are not at the right stage in life to be raising a child, even though their bodies can produce one. Someone who has finished school and is working hard to sustain a family is much more fit to raise a child than a teenager that is not ready. This is why it is a good idea to give babies up for adoption when you have one without being ready to be a parent so that people that are ready to be parents can have a child.
I hope that this helps you out! Have a wonderful day! :D