Table Designer & computed colums & case statement

i am creating a table using table designer and i want to add a computed column for Age as follows:

CASE WHEN BirtheDate IS NULL THEN NULL
  WHEN DATEDIFF(yy,BirthDate, CURRENT_TIMESTAMP) >50 THEN 'OLDER'
  WHEN DATEDIFF (yy, BirthDate, Current_Timestamp)>40 then 'middle age' 
  else 'younger'

END

however, whenever i type this code i get an error. is there something that i should change to get it work?

is it possible to use case in table designer for a computed field?

Any answers i would be grateful.

thanks

sukai