I bought an aging tool and poster through QDMA, something to checkout / buy, and yes the jaws can be tough to age, you usually can age them within a year ie he's either 2.5 or 3.5, but certainly not 4.5, so this can be good if you thought he was 2.5 and turned out to be 8.5! On the hoof, there are some characteristics to look for, which once you learn them do help, though most deer i see are .5-2.5yrs old. But it's nice to know, also being able to tell the difference from a button buck and a doe, and a button buck from a doe fawn is helpful. I'd say checkout the QDMA website.