Ski,
These two links from Indiana are great reference material. After looking at hundreds of jawbones from a certain locale, it becomes somewhat of an art. To make the best guess, I like to have both clean jawbones, preferably in my hand. Also, after 2.5, tooth replacement and wear method (TR&W) typically guarantees a "minimal age", thus why you sometimes see a "+" after most age guesses that are 3.5 and older, indicating the deer is likely that age as a minimum, but could be older. As with most things in life, there are always exceptions to the rule (of thumb).
IN Deer TRW Method Aging Info
Field Guide to Aging Deer
[/QUOfamily?
Thank you!