I don't care so much about the age difference, as much as the age.
16-17, she's still in high school and just a kid.
18+ (even if 16-17 is "legal" in many states), she's in college or at the very least, out of high school.
I won't have to tell my friends, "Here is my girlfriend, I have to drop her off back into her cradle by 9pm."
The only people who will judge negatively are other women. Guys will be like, "Dude, how much money does that guy have to be dating a girl that's like half his age?"
I don't care what my parents would have to say or think. Why would I date someone that young? Because I want to feel alive and experience excitement again. Because I'd want to escape feeling serious and responsible all the time. Because I'd like to feel someone who is still exploring life, and be a sort of tour guide for.
It's fun, but not sustainable. Sooner or later, she'll want to grow up and want adult things like having kids and getting married... which is stuff that I would have been walking away from and enjoying that I was escaping from.