Don't immigrants need a statement against immigration?

No need. 18 years old has reached adulthood and has full right to make decisions. In addition, it should be noted that even if a minor, if a party has no custody, it does not need the consent of that party.

Consent required: If you have custody, minor immigrants must get the consent of their parents who are not with you. The party with custody should write a statement agreeing to the child's immigration and sign it in front of a notary.