Lol. 16 year olds are boys. Most of them are still growing and definitely haven’t developed any significant muscle mass yet. It would be murder to let them fight 30 year old men.
Also, most fighters start training in MMA or other full contact combat sports around that age. Most sane people don’t send their 7 year old boys to MMA gyms to become cagefighters, lol.
You need at least 3-4 years to develop any martial art to a high level. Because we have 3 different areas in MMA- striking, wrestling and BJJ, a fighter would most likely peak in their skill set in about 9-12 years.
Given that most fighters start training at around 16, they will most likely hit their prime around 25- 28 and most will continue to develop and gain experience till around 33- 35.
The average, high school sports playing 16 year old "boy" would beat the average 30 year old "crossfit" man in a fight. The 16 year old will be way more physically explosive. If 16 offends you up it to 18. I'll repeat: the average high school varsity athlete, would beat the average 30 year old "in shape gym goer" in a fight; will have way better recovery, way better reflexes, and more natural explosiveness.
For fighters, yeah fighters need to acquire skills. But this is purely physical I am talking about. Let's say fighters started training as little kids like the do in the Caucasus and acquired the skills early. THE physical prime as far as bone structure being mature (yeah there are some late bloomers that take longer but most boys have mature bone structure by the time they are 16 or 17), healing ability, explosiveness, heart strength, nervous system etc will be the ages of 16 to 24. The reason most people here say fighters "peak" in the late twenties or early thirties is because they had to spend their entire early twenties working their way through the ranks before they can finally get to the championship level. most fighters only fight two or three times a year a most. That and the skill development can offset the at first small losses in physical abilities in the mid to late twenties - especially given that most western mma fighters don't start training as kids but instead much later in life.
I know, people don't like to hear this. the mostly thirty or forty something year old men on this board don't like to hear this. but it is fact. And, by the way there is nothing stopint 16, 17 and 18 year old males from putting on a ton of muscle and fast. They just need the proper training regimen and a caloric surplus diet - which being in school tends to make difficult. But there is nothing physical stopping it from happening and happening fast.