If you need to search, the easiest option would be to have an age attribute which you update daily. If you just need to display specific ages, you could use XPath, take a look here for tokens like [%YearLength%].
You XPath constraint would look something like (note that I did not validate the syntax):
[DateOfBirth < ['[%BeginOfCurrentDay%] - 8 * [%YearLength]']]
[DateOfBirth > ['[%BeginOfCurrentDay%] - 12 * [%YearLength]']]