Depends on where you live. The age of consent varies from state to state. Some states are 16 while others are 18. Check your state. If your state is 18 and the relationship becomes sexual the 18 year...
In some places yes, with the permission of the parents, who normally have to be on the marriage license, other places you have to be at least 16 even with parental consent. But remember marriage age...
There are no laws about dating. However, a 15 year old is under the age of consent and the 18 year old is an adult. Depending on the laws in their jurisdiction, should sex take place the 18 year old...