Derivative beneficiary children are only eligible for benefits from an alien's petition up to the age of 21. Its 21 and not 18. There are some other immigration issues regarding a child's age of 18, however, the most frequently cited issue is the accrual of unlawful status after the age of 18.

Under family based petitions, an alien's derivative beneficiary children are eligible to adjust status as long as the petition is filed before the beneficiary children turn 21. The CSPA ensures that only filing of the petition is necessary, whereas before CSPA, approval of the petition was necessary to ensure that beneficiary children would benefit from an alien's petition. The same is true of employment based petitions.

Your father's girlfriend is NOT eligible to petition for your behalf because as laneDear said, the stepparent-stepchild relationship will be created after you've turned 18. A stepparent counts as an immediate relative of an alien and thus U.S. citizen stepparents are able to sponsor the alien children of the spouses WITHOUT having to adopt the children as their own.

In order for U.S. citizen stepparents to sponsor the children of their alien spouses, two conditions must exist:

1. The step-relationship must have been created before the child was 18 years old.

2. The child is still single, and under 21 years of age.

As I've said before, CSPA ensures that the second condition is satisfied as long as the petition is filed before the child turns 21 years of age.

In your case, you would have to wait until your father obtains his green card before he can sponsor you. However, do not get married. If you do, then you have to wait an additional 5 years till your father obtains his citizenship so he can sponsor you. Unless you get married and your spouse sponsors you.