I don't like the looks of that at all, and I would seriously recommend that you have him seen by an experienced reptile vet as soon as possible, especially because of his advanced age.
"Tail Rot" , which is how most beardie owners refer to it, can be caused a number of different things......an injury to their tail, such as getting it caught under something in their tank, or a wound of some sort, such as their biting their own tail, or having it bitten by another dragon.
However, the most common cause of this problem is their having retained shed on their tail that does not come off as it should. When this happens,the retained shed on the tail begins to shrink, and, as it does, it begins to restrict the blood flow to their tail which causes the affected tail tissue to develop a serious infection, and the tail becomes necrotic, similar to a person who develops gangrene.
If I had to hazard a guess, I would say that that is what has happened here, since when they get up in age, somes they don't shed properly, as they should, which can cause a serious life- threatening health problem for them. If that is what I'm seeing, and I'm fairly sure that it is, then that portion of his tail will need to be amputated and he will need to be put on a very strong antibiotic, such as Baytril, in order to save his life.
I have been through three amputations with my oldest beardie, several years ago, and my vet eventually had to remove all but about an inch of his tail in order to stop the spread of infection. However, with a good vet, and proper care,he can recover fom this.
Ming was about 7 years old, at the time, and went through 3 amputations in a 5 month period, but he did fully recover, and is now nearing 10 years old, and still going strong, so there is hope for your little one's recovery, but DO NOT fool around with this.......GET HIM TO AN EXPERIENCED REPTILE VET IMMEDIATELY ! Please contact me via a private message and let me know what the vet has to say. I will be keeping him (and you) in my prayers!
Denise Bushnell