Why does age matter in a fight?

I don't think it really starts factoring in until you get around 35. That's when I first starting saying "I'm getting too old for this". That's when things like injuries and burnout start kicking in. As a 20-30 year old, I was indestructible (or I thought), and I could train all day and never get injured no matter how hard I went. Now If I injure myself by overtraining (which is very common now), I have to take about two to four weeks to recover, so my cardio and strength deteriorate.

I think knowledge how to fight always beats no knowledge. If the 21 year old trained to fight, of course he will have a much better chance to beat an older or any other guy for that matter with no knowledge of fighting and if the older guy knows how to fight he will beat the younger guy. In fighting there are never any givens and no guarantees for anybody.
Why a 21 year old with fighting knowledge would want to cowardly beat up on an older or any guy with no knowledge of fighting is beyond me unless he is a criminal as this would be assault. The person who does not know how to fight would probably not willingly agree to a fight.

age doesn't matter ,age is a factor ,
mike the hammer Tyson was at his peak at 21
the body literally "decomposes" after a peak age ,which is different for everyone
26 is a good round number
why do u think people take dick pills and hgh in their early 30's

Being a senior citizen I can speak first hand to this probably. I am still quick for instance but my reflexes have slowed somewhat from what they were before. Then there is the healing process and while I can still take a shot and some punishment it takes a lot longer for me to recover or heal up from aches, pains, strains, etc. than it once did when I was younger. Strength wise I have lost about 20-25% of my strength and at one time I wore a size 44-46 coat and 30 waist pants. Now I wear a 44 inch coat and 34 waist pants. Still many people think and believe I am 20-25 years younger because of the shape I am in and my appearance but I myself can see and tell the difference even if they can't. I will also add I don't train or workout anymore. 25 plus years of that were enough and so now I do other things for fitness like mow my lawn with a push mower, or maintenance on my home or help others with such things.

You don't get it because you think it is all physical. Has it ever occur to you that an older fighter has this thing call "experience"? The thing about the young is they they concentrate so much on the body and rarely if ever that thing between the ears.

If they're about the same weight and strength, then the 30 year old will have a third more experience at fighting.