How do I create a check constraint that is based on age difference?

Hi all,

How do I create a 'check' constraint on a date column, so that it will only
accept dates of at least 28 years ago from the date of entry?  For example,
only allowing dates of birth of people who are older than 28.

i.e.

create table foo (name varchar(40), dob date check(date('dob') < date('now' -
28 years)));

This doesn't work.  I don't understand how to do addition, subtraction, etc
with date fields.

Regards and best wishes,

Justin Clift
Database Administrator