There are few things to consider to understand why Google Analytics not showing any data under Demographics over view for your website.
First thing first
Please verify that your code is rightly implemented & Google Analytics states that it is ‘Receiving Data’ under Admin’s Property tab
If your Analytics code is working right & implemented correctly then following things you should check if those are the issues you are facing…
You may have vary less traffic that Analytics doesn’t have sufficient data to show you.
Another thing is that
Probably Google needs sign in search users to their website to gather & guess their age & gender through their profile & to show it in demographics overview tab
So right thing is to consider giving & designing right content for your website to maximise web traffic to your website through google searches with right tactics of Search Engine Optimisation i.e. SEO
Hope it helps,
Display Advertiser Features i.e. Demographics and Interest Reports & Use enhanced link attribution not working Google’s new Universal Analytics, Here is how to fix it & make it work & show relevant data by tweaking tracking code
Thanks & Regards