Andrew, here are 2 confirmed old bulls without tufts on their foreheads. Look especailly at the first one, it is completely blue from age related hair loss. Both where taken in the same region of Namibia, bordering Botswana.

In contrast, here is a pic of a youngish bull I shot for meat, not more than 150km away from above mentioned region, but already showing signs of nice hair growth.

And then some "classic" mature bulls: