Legally, I'd argue that there shouldn't be any age for either males or females. If you have a moral opposition to your children viewing porn, then its your responsibility as a parent to regulate if/when they see it. Don't expect the government to do your job.
As far as the question of girls viewing porn specifically, I'm going to buck the trend here and say that parents should set a higher age for girls to view porn than boys. Body image problems among teen girls are rampant. The last thing that girls today need is to see the absurdly unrealistic body of the average pornstar and be told through the content, "Unless you look like this artwork of plastic surgery, you'll never be sexually desirable." Its an issue that boys face far less since, 1) body image problems for boys aren't nearly as common as they are for girls, and 2) many of the men in porn are old, overweight, and butt ugly.
I'm certainly not going to say this is a universal rule, which is exactly why it should be regulated by parents, not government mandates. I'm sure there are plenty of girls who are capable of distinguishing the artificial reality of porn from the healthy goals of real life. And I'm sure the age at which they're able to do that is different in every girl. But as a rule of thumb, I'd say the issue of girls viewing porn is a bit more complicated than it is for boys and demands waiting a bit longer before they're exposed to it.