Most Helpful Girl
Depends on the woman.
With me it doesn't matter so much about the guys age (unless he's underage), but more so if I find him mentally and physically attractive and how he treats me.
My current boyfriend has recently turned 18 and I am 19, 20 in October.
I don't like him just because of his age, it's because I find him cute and he is nice toward me, caring, has a good sense of humour, seems to learn from his mistakes, is respectful of my beliefs and seems to be honest and loyal from what I can tell.
Some women may have a preference for younger or older guys, but personally, I have liked guys both older and younger than myself, as well as guys about the same age.
As I say, age doesn't matter too much to me so long as no one is underage and if they are, just don't do anything inappropriate with them.
I also have friends who have liked guys both older and younger than them too. One of my friends is actually engaged to a guy 2 years younger than herself.1
Most Helpful Guy
It's hard to describe what "usual" is. From my limited experience (flawed sample, to be taken with a grain of salt), it's actually considerably more common for older women to date younger men.
I see that a lot in Japan where women past the age of 30 start dating younger (sometimes considerably) boyfriends. It is much rarer to see, for example, a 25-year old girl date a 19-year old young man than it is to actually see a 35-year old woman date a 19-year old young man here.
Why exactly, I don't know. I don't want to offend or risk inaccuracy by guessing. And again this is just from my narrow and limited life experience.1