depends how long you've lived, where you've worked, the state of your social life, how well travelled you are...I don't have anywhere near that but as an older person who (shock horror) didn't grow up with the Internet, email and social networking, I have lost contact with an awful lot of people who I know today would be 'Facebook Friends' in the most modern of circumstances. I don't search for people I have 'lost' (I have a life, mainly, but lots of people marry, change names, work in professions where they would rather clients couldn't see them etc etc etc.) but occassionally I accept a friend invite from someone I haven't seen in 15 years. All good.
So, in my opinion you are being entirely unreasonable unless you know that the person concerned has never been to school, never had any friends, has no family and has never left the house.