I think that's pretty disgusting. At that point, the 18 year old boy is most definitely taking advantage of the 13 year old girl whether or not either of them are cognizant of that fact. A difference of 5 years when you're that young is the world. Now if you said a 20 and a 25 year old, that would be a completely different story! Honestly though, I am even skeeved out by my friend that married a guy 15 years older than us because we're only 22!