technically yes but of course you do run a risk, at least if you are the eighteen year old. While you may only be a few short years apart in age she or he can have you locked away if you ever pissed them off, because obviously if you are in a relationship you will kiss and such forth possibly leading into intercourse. Now if the other person does however become mad they can say that you have molested them, which the dna will show has happened and that my friend is some jail time. However if you can prove that this has been an ongoing relationship for several months or so you may be able to get away without any charges, it just depends.
it is stupid i know because if you honestly look at some of the reltationships out there, they have more than a three year difference at least most of them do.