As to your more recent pic nearest this post, I'd just have to say 5 1/2 or older based on the pic,
while wouldn't be surprised to learn he's known to be 7 1/2.
Every year older, there is significant loss in accuracy,
with a bent towards the deer being older than they appear.
Much like humans, individual deer each have their own genetics for either good teeth or bad teeth. Some individual deer's teeth will just wear much faster, even in the same cohort.
You could have two bucks which were brothers, born to the same mother on the same day, and one's teeth might look 3 1/2, the other 5 1/2, yet both might actually be 4 1/2.