I thought age had something to do with it, but as my character on Beluaterra fought lots of huge and long battles in the last invasion as a teenager and never recieved an increase I thought they had been removed entirely.
My East Island character that I started with recieved leadership skill increases in just about every battle he fought, until the point where his skill couldn't be measured at the academy, so I thought that if they hadn't been removed the odds of getting one had been lowered to the point of them being negligible.