That actually doesn't have to be a mystery.  You can check everything it thinks about you, including age, gender, and interests.  I'll edit the URL in here if I can find it again.

ah here it is https://adssettings.google.com/authenticated?hl=en&utm_source=privacycheckup&utm_medium=web&utm_campaign

I think this is a generic link that will take anyone to the settings for their account

Umm, this was pretty high up...

Google, you're scaring me.

While not all of it is 100% accurate, its amazing how much they get on you.

Personally none of this worries me too much, so long as the information isnt getting into stuff most people shouldnt know like actual home address (the town i live in is fine so they can give me local businesses) or my Full name, birthday and Social Security Number. 

It's the trade off of privacy or convenience, and striking a balance between the two. (Though I do run adblockers LOL)