Based on research (I know... some of y'all don't go for that "sciency" stuff) These measurements are typical of known age gobblers in spring:

less than 1/2 inches is jake
1/2 - 1" blunt is 2 yr old
1 - 1.25" pointy is 3 yr old
over 1.25" sharps s 4 yr old

In a 50 bird study of know aged wild turkeys in Missouri, these were the average spur lengths
jake - .25"
2 yr old - .87"
3 yr old - 1.01"
4 yr olds - 1.08"
5 and older - 1.25"

Spur length and sharpness does have a lot to do with terrain and soils as well as subspecies. There is a lot of variance even in the same population and I have killed legitimate jakes with 3/4 " spurs and weighing nearly 18 lbs, adult gobblers with no spurs and a beard weighing 15 lbs, also no beard and spurs. I can handle one with spurs and no beard but a beard and no spurs is a definite foul on the play. I agree with Yeks observation that head size and cap thickness seems to increase with age - body size most defiantly does NOT - the oldest birds I have killed were smaller.

Beard length is more a function of leg length, beard thickness, and abrasiveness of the vegetation the gobbler feeds in. They get to about 10" in their 2nd year and wear off from there. Beard thickness has no function relative to age - it has x number of beard filaments when it is born and no more and no less as it gets older.

My $.02

Ive always said beard and weight have zero to do with it. I have killed a few jakes that way 16lbs, but adult gobblers that I would say were 3-4 + years old that only weigh 14lbs. Those small gobblers are what I call "Indian birds" and some of you have called them "Mossy heads." I have always thought it was just a different genetic strain of wild turkey, and probably the original wild turkey. The amber tips are not significant in aging either, although the jakes do have them. I have killed many 2 year olds with out amber tips. Not sure what it is, I always thought where it was part of growing, or beard blight, which I have killed a few also.
To me, typical jakes have a 2-5" beard that is "stiff" and sticks straight out, do not have full fans, have "nubs" for spurs, usually about 1/2" or less, their legs and feet are almost black, and their primary wing feathers are different.
2 year olds have a full fan, primary wing feathers are developed, have about 3/4" dull spurs, and a beard anywhere from 8-10"s, which the beard has nothing to do with it.
3 year old, spurs are about a 1+" long, with a little curve and failry sharp.
4 year old, 1-1/4" spurs with some good hook and sharp
5 year old the birds that are seldom killed with 1-1/2" spurs and hook like a game cock, and sharp as a tack.

This is what I go by on the norm, and nothing else will change my opinion on it. I do know in mountainous rocky terrain will be different than the swampy or sandy areas.

Really good info. What about the differences in spur color. I've seen spurs over an inch that were almost pink with a darker tip, and some that are entirely dark. Is that just another genetic deal, or maybe a melanin issue?