How can i change the filter of a computed field to my requirements

i have created a computed field which calculates the ages from there birthdays and when its exposed it gives the following filters :

* Is Greater Than
* Is Greater Than Or Equals
* Is Equal To
* Is Not Equal To
* Is Less Than Or Equals
* Is Less Than

and a text field where we gave the value for search

Now my issue is that i want to display the ages according to my way like this :

6 & below
7 years
8 years
.
.
.
15 to 18 years

so please help me how i should i do this...

thanks