To set age ranges for selection during booking, open the Booking tab and go to Age Ranges.
The column on the left is the value sent to the booking system. The column on the right is the value displayed in the booking widget drop down.
Once you have configured these columns, go to Booking > Booking Buttons.
Select "Add Layout" and then "Renter Age" and click "Update".
You can now preview and test the drop down on your staging site.