well assuming your not a 20 something year old hanging out with middle schoolers, I don't see the big deal. I think as long as your all over 18, the age rule kind of goes out the window. I even hung out with my roomates cousin a lot and he was still in highschool I was probably 19...but we hung out with his parents too...everyone was cool and it was just a bunch of stoner video-gamming geek's.
In fact I think it's generally acceptable for any highschoolers to mingle regardless of what class they are in. I've seen plenty of seniors date freshman and sophomores. And I dated a senior when I was a freshman....