Its not illegal to be 'dating' and 15 year old if your 19. But if you so much as kiss her you can be considered a sex offender for the rest of your life.
Having sexual relationship with a minor is always illegal.
As for dating, there is no way to prove that 15 year old girl and 19 year old girl are in a romantic relationship and even harder when both...
There are no laws about dating ages. However, sexual contact is likely to be illegal in most places, as the age difference is more than 3 years and she is under the age of consent.
I am in the same boat...so i think that if both parents are okay with it and there is no sex involved...then it is okay. but...there are those concerns of other people who might turn you in.