search between two table fields

I like the visitor to search for events for his age.
I have a text field where he enter his age. In the table I have a MinAge
field and a MaxAge field. I can filter the record with:
WHERE EMinAge <= varAge AND MaxAge >= varAge

This function, but how do I get all the records listed while there is no
etry in the search text field? Right now I get zero records back when this
field is empty.
I think it is something with the Default Value but I can't use the % sign.

Many thanks for your help

Esther