When I was a grad student I taught some psych classes in the nursing faculty. Of course, I was 21-22 at the time so I had a fucking great time!
I was teaching high school here in Germany a little while back (where the students are often 19 or 20 in their final year) and now I'm not 21 years old any more. All I can say mate is 'eye contact' - and keep telling yourself you're old enough to be their father.
Or wait until they're not in your class anymore.