I'm not sure why none of the "show data as" options didn't work, but you can do it by creating a 3 "New Calculated Items" on Gender.
The calc would be 100*'Female'/('Female'+'Male'+'Not Declared'), with the same for the other genders.
Then, go into the Gender column properties and create a conditional data format for when Gender IS NULL. Make it percentage, with however many decimal places you want.
EDIT: Sorry, I skimmed too quickly. The above gets you gender pct by age bin at all subtotal and grand total levels. It looks like you need a calculated column
along the lines of 100*SUM(Workers by Region, Month, Age Bucket and Gender)/SUM(Workers by Month)
You're DA MAN!!!
Just one correction to the proposed formula:
Using: 100*SUM(Workers by Region, Month, Age Bucket and Gender)/SUM(Workers by Month) calculates same percentage in all rows.
After small correction: 100 * Workers / SUM (Workers by Month) calculates as required.