There are a number of free short stories available here on my blog. One of the most popular is "You're Never Too Old For A Spanking" which features an older (40-something) heroine.
Here's the link to part one.
I also have an older heroine, younger man combo in my book Laying Down the Law.
I like writing about older heroines, I suppose in part because I am older myself. I think they also have more flaws and interesting backgrounds. Plus, I don't think that younger women, with perfect bodies and flawless skin, should get to have all the fun.
What about you? Do you prefer heroines of a certain age or does it depend on the story?