I haven't heard of any law which prohibits an 18 year old male from dating a 14 year old female. However, I guarantee that all states have laws which prohibit any form of sexual contact between them. And that can include simply touching each other.
All US states have age of consent laws which establish the age at which a minor may enter into consensual sexual relations. Parental consent in issues such as the age of consent is not relevant as parental authority cannot supersede existing laws.
Remember, anyone can report the two of you "dating" to the local Family and Children's Services Center, who might become very interested in what an 18 year old boy sees in a 14 year old girl!