A jake is probabaly about the only for sure Turkey that can be aged very accurately.
It gets alot harder after that. A 2 year old will generally have 1/2 to 7/8 spurs but that varies and is definitely not a true indicator to age.
Anything bigger than that is truly just an educated speculation of age.
Jakes are easy to identify with beards 7” or less and spurs just being nubs.
2 year olds are next with beards anywhere from 8-11” and spurs generally in the 3/4”-1” range.
After that, it’s a crap shoot. Beard length won’t really gauge anything (the heavier the beard, the older). Spurs ranging from 1 1/8”-1 3/8” and you’re probably looking at a 3 year old. Anything over 1 1/2” and he’s an old bird.
Gobbler age is not necessarily a science. Jakes are obvious but beyond that it can be difficult. Now, generally but not always, a 2 YO will have rounded spurs 5/8" to 3/4". Beard length doesn't matter much. Where you really have to be careful is assigning spur length to age. I still remember a sidebar re:turkey age I saw in a turkey magazine years ago. Back when MO was trapping and transferring they tagged a fall jake and released him in another county. At that time, you had to bring all bagged turkeys to a check station. Well, that gobbler was killed later and checked in. At the check station he had a 9 1/8" beard, weighed 19# and had 1 1/8" spurs. Per his leg band he was 9 years old. So moral of the story: jakes are obvious, 2 YOs are pretty much obvious but not always, beyond that they're just mature adults.
Have said this for years. A lot like a bucks antlers, spurs don’t tell the whole truth when it comes to age. Hunted a bird for three years that had one white tail feather. He got shot by neighbor and had a 9 inch beard and barely 3/4 inch spurs.