I've been surprised about how much girls have cared about this in the past.
I'm a guy and I wouldn't care if a girl was a few years older. But I've previously heard some very intelligent girls say that they would absolutely hate going out with a younger guy. I remember a couple of 21 year old girls saying that going out with a guy a year younger than them (the guy was one of my friends) would be like "dating a little schoolboy", which I found quite surprising and almost disrespectful, especially since the girls were highly intelligent Cambridge students.
I guess it depends on the girl - some will care, some won't.