We are not to be unequally yoked. (2 Cor. 6:14)
A man must provide for his family. (1 Tim 5:8)
One must leave their parents and form a new family. (Gen. 2:24)
All of these require a good bit of age and life experience. Especially in the matter of being equally yoked, one must have had the time to really test and work through what they believe and why. After this, they need enough life experience to be a good judge of character. This is rarely accomplished sufficiently in a younger individual. I can say from my own experience that when someone is not out of their teenage years, it is unlikely that they will be able to fully make these decisions properly.