Cos at the moment our profile asks for our age, amd for those of us that like to be honest about our age we have to remember to change it when we have a birthday.
If we had the option of putting our DOB in the age could be calculated automatically, and would always be accurate like for our DC (I am assuming you do MN speak ).
There may be others that want to keep the 'age' field the way that it is so they can be forever 21 (or whatever) - so the option of either might be nice.