I age them by spur length only, as long as their an adult as far as feathers are concerned.
I know what chamberlain says. He's probably right. But just for my own fun I age them, always have and always will. (Regardless if it's right or not it's just me)
My personal way...
Jakes=nub Spurs, usually not exceeding 1/2"
2 year old=usually blunt with no curve. Between 1/2" to 7/8"
3 year old =usually curved and pointed, 1" to 1-1/4".
4 year old =very curved and very sharp, 1-1/4"+
5 year old=extremely sharp and curved, and if you throw it over your shoulder wrong you'll gash the flesh between your thumb and index finger. 1-1/2"+
Again, this isn't scientific at all and I admit that. It's just my way that I do myself. Its somewhat like having a big imagination as a kid.