How about a nice 36 year old Gag guy who could teach her about life?
Lol, just kidding. She should find someone close to her own age, of course. I would say, at that age, when you're just leaving home, VERY close to your own age. By the time you are 25, a man even 35 would be fine, but at 19, he should in most cases be within a year or two of your age.
Honestly, it only matters what you're comfortable with. You'll likely find that guys your age are immature so naturally you'll look at older men. Then you have to ask yourself if the stigma of dating an older man is something you're able to ignore, and whether or not friends/family will give you crap about it or be supportive because they want to see you happy.
I'd say around my age and I'm not saying that just being thirsty. I usually date younger and girls usually date a bit older. Always seems to work out that way and 19-23 isn't that far of a gap for it to be weird. 25+ it's going to change the dynamic.
The rule is half your age plus 7, and that's the youngest you can date. So as a 20 year old, half your age plus 7 is 17, so you can date 17 year olds. The reverse is your age minus 7, multiplied by 2, so that'd be 26. 17-26.
17 - 24 is probably a good age range. You could go older if you really wanted to, people would probably look down on you guys though. I wouldn't go younger than that either though I'd doubt you'd want to.