Quite frankly it would make answering questions a bit easier if you know the person's age. It lets you know their maturity level/ an estimate of their social skills. We all know that relationships are different in your twenties and thirties than in your teens and so is dating in general. For an example when anonymous teens are asking for date ideas I would like to provide a more suitable answer than "go to a bar, etc." Also its really awkward on the sexuality section not knowing the person's age at all. Sometimes I have an idea of their age due to wording but quite frankly sometimes I don't know whether I'm giving sex advice to really young teens or a really naive adult.
Can't remember the last time I used anonymous. But if I were to do it and my age was given, would you have a single bit of trouble figuring it was me? there is a good reason for using the age range rather than exact age.