manipulate the data in a 'search'

Hello everyone.
How can I manipulate the data in a filter ‘search’ in the grid?
example:
filter:
Birth: from date … to date … (database field ‘datanasc’)
age: from … to … (new temporary field)
The user could enter the date 01/01/2000 to 31/12/2014 to refer to all those with an age between 1 and 14 years
or
the user could enter (for his convenience) from age 1 to 14 years to refer to all those born from 01/01/2000 to 31/12/2014

If I indicate the date of birth is ok, since the field in the database, but if it indicates the age I should build programmatically from date … to date …, but can not find the reference fields
{datanasc} is the field in the database, but … the second value (the date) as the check?
… where datanasc between 01/01/2000 and 31/12/2014 : I know only filed ‘datanasc’, but …datanasc2?
Hello
Thank you
Henry