Green horn, you asked if the first "dark ring" is the 3 year ring.
I am going to try and help you out with this.
I am no expert, but I am a fanatical BIGHORN sheep photographer.
I spend a ton of time out in sheep country. I have even been asked to help out on a couple sheep hunts.
In my experience, the three year ring is always hard to see.
Example, see this photo. There is, what I would assume to be a three year ring, just above the busted section of this rams right horn.
If you compare its visibility to the visibility of the other growth rings, you can see that it does not have the same hardness to it.
From distance, the three year ring can be tough to see. Also, especially on older rams, the three year ring can be totally broomed off.
Through my spotter, unless I have a good view, I default to assuming that the first ring I can see is likely the four year ring.
Another good rings image.
That being said.
You really have to spend some time with sheep to get an understanding of rings.
I don't actually believe that you can always go off of rings.
Ever heard of "rut rings" or have you ever seen a sheep that appeared to have lots and lot of rings?
They sometimes only space a half inch apart.
I have also seen rams that appear to have almost no growth rings. Its not common, but sometimes the rings are so faint, that seeing them from ay kind of distance seems impossible.
So how would you "age" those sheep when you can see a mature ram, but only see say one or two rings?
Sorry, I don't want to make it harder for you.
Like I said, I am no expert. Just a fanatic.
Growth rings are the best guessing tool we have.
But they are not always accurate.
Genetics, seasonal conditions, food and water all play a role in your areas sheep herd.
They all play a part in horn growth as well.
It is possible that in your area, the three year ring is a hard, dark, very visible ring.
Not typically in my area. The only way for you to know for sure is get out there and look at a lot of sheep.