???
What was harder? The boys? That's because their drive diminishes in the 30s. ;)
The research was conducted by Alfred Kinsey, if you are interested. A lot of it has to do with the fact that women in their 30s orgasm more often than in their 20s, but this may be due to the fact that women in their 30s are more comfortable in their own skin, have more self-confidence, and have learned how to make sex more enjoyable. Their lovers tend to be more skilled, too. They don't call them the "dirty 30s" for nothing.
Here's another theory: [url=http://www.time.com/time/magazine/article/0,9171,2007422,00.html]Study: Why Older Women Have Higher Sex Drive - TIME[/url]
I think if you are in your 30s, it's a matter of use it or lose it, but if you are using it, you can never get enough.