I noticed a network profile of a user with the age shown as -972:
I dug further to see if there are more such cases but my analysis shows that this is a lone case, though there are a lot of users who have not added their age to their profile. I believe it is a bug in the system that allows the users to enter negative numbers as their age.
I emailed Stack Overflow team about the same and I was suggested to use Meta for discussing it further. I would like to know if this is indeed a bug or there is some explanation behind this.