Is there any state in the United States that will marry a 15-year-old to 21-year-old if they have kid together?

No, there is no state in the United States that will marry a 15-year-old to a 21-year-old, even if they have a child together. The minimum age for marriage varies by state, but it is typically 18 years old. Some states allow minors to marry with parental consent, but the minimum age is still typically 16 or 17. In addition, most states have laws that prohibit adults from marrying minors, even if they are pregnant or have a child together. These laws are in place to protect children from sexual exploitation and abuse.