What is the trend of gender distribution among pilots?
At least for the FAA, you can get the U.S. Civil Airmen Statistics as a download that covers from 2005 to 2014. It includes data based on gender and license type. I'm not sure when they will update for 2015. If you download the 2010 one, it has data back to 2001, so about 13 years of data.

Here is an example of the data provided:



Similarly for the UK, you can download statistics from this website. I haven't downloaded them but it looks like what you are looking for is there.

You may be able to find similar data for Canada, Australia, and some other relatively "open" countries.