I've measured a couple over the years but not enough to get an average per-se.
This buck was a 3.5 year old that a lease hunter took on our place and was right at 16" ear tip to ear tip.
Where this one was 14.5"...
don't get me wrong, I still think this was a good dear, but he had the worst case of ground shrinkage on any of the deer I've taken. He was taken on the same property as the first deer with 16" ears.