[Haskell-beginners] Type constructors sharing a common field

[Lorenzo Tabacchini]

If you could have a single polymorphic "age" function for all types 
having an age, you could never have this runtime error, because the 
compiler would infer which "age" we are referring to.
The GHC OverloadedRecordFields extension that Michael linked seems to do 
what I am looking for (http://www.well-typed.com/blog/84/).