Not a major issue but worth noting anyway.  When Admin chooses setting to display birthday as "age", the profile should say "age".

I don't think user should be able to hide their age because people need to know who they are talking to. (Or at least admin should have control whether they allow users to hide age.) 

I think it's better if there is 2 fields on profile, one field to show age by default and another to show birthday if user wants to display date publicly.