A child may be mature enough to get married, and it should be his or her choice, BUT, as all parents know, sometimes when a younger person is eager to get married, he doesn’t always see the bigger picture. Sometimes the IDEA of getting married,especially if all of oen’s friends are busy getting engaged also, is more appealing than actually doing so. Many girls feel pressured into getting married before they really know what they want out of life. They have been structured into thinking along certain lines, especially in seminary, and when they see what real life entails, they are shocked. Sometimes a parent sees something that is not evident to the shadchan or even to the boy or girl involved. It might be something as subtle as noticing something about the family when they meet, or as blatant as a real chisaron in the boy or girl.
If the objection is because “the other side” is not sufficiently wealthy or yichusdig to suit them, then I hope the child does not allow that to interere.