I would prefer to date someone two years older than me because I want to start talking engagement soon. And I don't think a teenager is ready for that. So, odds are I wouldn't get what I want and wouldn't feel totally satisfied.
Also, if he's older, then he's probably a grad student with a few years to go as well. So, more time to hang out. On top of that, he's probably more established.
I would date a 19 year old. But I would rather date someone a little older, especially since guys often take a little more time to mature, and I want something serious at this point in my life.