Most full grown adults weigh more than teenagers. Since mass + acceleration = force, a heavier person's punch or kick carries more force than a lighter person's. More force equals more damage to the face and the body being struck. Second, adults are allowed to consume alcohol and alcohol leads to more fights and "more brutal" fights since each fighter has less inhibition about throwing a punch than a sober person has. Adults also have greater access to illegal drugs like PCP which make them impervious to pain. I would hate to fight a person on PCP. It would be worse than any nightmare I have ever had.

Not most of the adults are like that...Most, if not all of the ones that I know, my age, older, or younger, are not like that....(Kicking in the head someone while he is unconscious for example)

Some young adults, are like that, because either they haven't fought for a long time or never, or got bullied a lot as kids, or are having some issues, or are cowards, or for many many other reasons, so when they do it, it may escalates to something like that...

Kids and teenagers are fighting more often so they can control more the situation many times, they are more innocent and something similar goes with many adults, that either they are mentally healthy, either they have a lot of successful self-defense experiences combined with a healthy mentality, ether they are secure enough ,either something else similar or a combination of those.

Plus, the vast and one sided brainwashing propaganda, that it is going on about safety issues, that leads some to go crazy any time that something happens, or even before, rather than defuse it or control it with ease....

Normally a healthy adult should be able to apply the appropriate force or defusing the situation in whatever the case might be, rather than go crazy and when done with it, to behave and feel like nothing has happened...

From the other hand, in order to do the last, you may be need to be able to escalate it, as much as it needs...So it might be like a coin with two sides....if you know what I mean.

Because they're not playing around and they mean to actually harm each other. It's one of the reasons that you should avoid fights at all cost if possible. Fighting should be a last resort as it often ends in injury.

Adults have more testosterone, power, confidence, and aggression.

Because they are adults and are much more powerful

whenever u see teens 13-16 fight

it usually results in a kick or a few punches to the head and a headlock bamm fight stopped

adults;20-45

usually ends up in a Knockout brain injury glass to the face

severe kicks to the head no remorse, why is this?