There are probably a few ways that you could do this. One way that I can think of is to take the values that are entered for the age range and convert the age range to a date range with some php code.
You can then do a search in your db based on the dates that you have. To do this with a DataAssist search you would want to make sure that you use the GET method so that you can perform your age to date conversion then pass the values in a query string to your results page. Then the DataAssist search on this page can use the date values that you passed in the query.