I don't know where you are, but generally, around 16 is the age of consent. So if you are 18, you're not in trouble unless your partner is 15, and the parents think you are sketchy. Which is pretty reasonable, id say.
What is legal is very different from what is right. In an ideal world, every last consideration should be sorted through to arrive at a right answer. Technically, an honest, open, responsible relationship between a 13 year old and a 30 year old could be appropriate. But realistically, i'm sure every 13 year old (and 30 year old who is interested in a 13 year old) THINKS that their hypothetical relationship should be considered kosher, and it's usually not. And by usual I mean that there easily could have never been such a couple that were actually responsible, and such a relationship is purely hypothetical. From a law perspective, you have to consider what can be taken as a blanket rule applicable to all people without bias or discrimination, that best balances protecting children with the "children's" burgeoning personal responsibility in regards to sexuality. For this, I think that 16 is a fair age at which you can fully consent. Younger then that, a couple can wait until the younger party turns 16. If they can't wait until the younger party is old enough to get a little more life experience, then clearly they do not have the level of responsibility required for such a difficult relationship.