This is one way to do it.
Create a dummy ad, pointing to any url at this Facebook Ads Create page - (you're not actually going to buy any ads)
Select the page subject as an interest - to demonstrate I've picked Shakira, the world's most liked person on Facebook
Here are the results for Shakira in the UK - it seems to make sense.
(Anyone else know a simpler, free way?)
Note - this may technically be the demographics of people who list Shakira as an interest, rather than page fans, but even so it must be a good approximation of their demographics