Testosterone is one helluva drug, man.
I know exactly what testosterone is. But it is hardly the only thing that makes an old man feel like an old man. There is alot of other stuff that happens to a body that makes it feel old with age. So much more needs to be taken into account then lower testosterone. HGH for example is considered much more of a fountain of youth then TRT.
So the "its unfair for a 40 year old experienced fighter to feel like a 20 year old." I always found a bit jumping the gun and a huge exaggeration.
Quiet frankly... there isnt anything thats going to make a 40 year old feel like a 20 year old. If there was that would be one of the greatest inventions in medicine. Hell you bet your ass when i turn 40 i would be on it and it would probably be the norm for most of society that can afford it.
I also kinda imagine they would be scared to release it to the public since so many older men would not be spending nearly as much money in medical bills as they are now.