So, MMA stands for Mixed Martial Arts. Mixed Martial Arts is just that. Mixed Martial Arts. The basic idea is they’ve been training a style (or multiple styles) to cover all their bases i.e. hands, feet, and groundfighting. They use fancier terms like “striking”, but that’s what it is. MMA is more a type of competition than a martial art in and of itself. Starting with “MMA” as a chosen martial art for your character is just going to confuse you and your research. The problem is that MMA is a culturally coded term for martial artists, professional fighters, and anyone into that kind of sport. The term gets flung around like a catchall, but the people who know will understand what you mean. This is where writers and those outside the community get tripped up because they think these martial artists are talking about a real martial art rather than an entire, vast subset of various disciplines from all over the world. Like any other culture, misuse of the term will automatically expose the author’s ignorance on the subject.
So, saying, “my character has been practicing MMA for ten years and participated in tournaments” will mean either, “my character is a professional fighter somewhere in their late twenties/early thirties” or “my character participated in tournaments where multiple martial arts were in attendance and the rules were changed to allow them to mix.” This would not be the whole tournament but rather a smaller (usually sparring or open form) division with others specifically for these various styles.
A character can be training in Krav Maga and Sambo and still be MMA, just like another training in kickboxing, jiujutsu, and karate will also be MMA. The idea behind MMA is anything goes, and they do mean anything. You could get collegiate wrestlers going up against brazilian jiujutsu artists with a side of tai chi chuan, and people might go, “weird, but okay”. A school might advertise themselves as MMA, but there’s always a named martial art underneath the generalities.
MMA as terminology is inextricably linked to the UFC, because… that’s where it starts. The idea behind the UFC and MMA in general was to create an arena bloodsport of “anything goes”, opened to martial artists of varying styles rather than sticking to older rules like the ones found in boxing. Rather than specializing in boxing or kickboxing, a fighter would need to be a master of multiple combat areas rather than being limited to only hands, hands and feet, or the ground. This is why it’s called Mixed Martial Arts as the fighters might need to branch out into other disciplines to fill in their style’s gaps.
Now, MMA has narrowed over the years and is on track to becoming a combat style all its own but it isn’t there yet.
The issue for you is going to be the UFC. If you were imaging some sort of no holds barred, competitive bloodsport for this character then they’d need to be 18 or older when they started competing. They’d also need a manager, a promoter, a sponsor, a team, and all the other ancillary details that come with being a professional fighter. This means that if your character started training when they were eight, then they’d be eighteen now and couldn’t have participated in tournaments unless they were the standard martial arts tournaments and those are far more limited in what they accept for categories. When we’re talking tournament, Mixed Martial Arts is just a code for “professional fighter”.
Due to the dangers involved, they don’t let kids fight in professional tournaments. They might let them outside the US, depending on legal age of adulthood, but you won’t find young teens in MMA tournaments where professional combat is highly regulated.
So, go pick a style that is under the MMA header and start researching that. Most of those specific styles will have tournaments your character could participate in under the age of eighteen, and (if they were good enough) travel around the country/world competing. Some of them even have Olympic options (Ronda Rousey is a bronze medalist in Judo), and a high school wrestling background is also on the table.
Ten years is certainly enough time for them to start volunteering as an instructor. Martial Arts assistant instructor is a common part time job for older teens in many martial arts schools, even below black belt. If you volunteer your instructors will often let older/more advanced students assist with the youngin’s or adults at belt levels below theirs. These are the students that assist with the classes, they do not run the classes, and at most will be working with smaller groups of students.
The role of the Head Instructor or Master Instructor is a full time job, so your character would have a much harder time competing and running a school at the same time. Someone who is heavily into martial arts competition will spend a lot of time (and I do mean a lot) practicing in their school, so they will be heavily invested in the culture and usually that means roped into assisting when there’s a large class on the floor.