Pages about persons where age is calculated automatically are saved in the cache until the next cache purge or the page change happens. Calculated value of the age is also saved.
After the date of birth of a person passes the saved age does not change.
Example: Paul McCartney has birthday on 18.06.1942. His wiki page was edited before 18.06.2013 and his age of 70 was saved in cache (which is correct). Then after 18.06.2013 no one changed his page or invalidated cache, that's why his age was 70 while it had to be 71 (which is not correct).
Notes: This can happen for all pages with calculated age. Possible, other calculated fields are also affected.
Possible solution: invalidate cache on some conditions met. For example, invalidate the cache of Paul McCartney's page on 18.06 of every year by settings Expires header.