Fast twitch muscle is basically just muscle composition, like if the child has muscle composition that tends towards the muscles working explosively in quick bursts or less explosively over a longer period of time. In other words, personally I can jump but I can run for like, two minutes before I get tired
None of the gyms I've worked at really have designs on anything more than level 10 and NCAA unless something exceptional works out (unlikely). From most of what I've heard about programs that do have an elite track, it isn't like they pick the kids out the first day they walk in anyway. I know a few girls who were elite tracked at regional gyms either currently or at some point in the past, and there it happened once it was clear they were progressing fast in compulsory and picking up optional skills (these gyms don't do TOPs). 8-10 years old. Gyms that do TOPs and elite track would I suppose essentially be going down that path around 7 if they tested the first year, but even that would be after several years in the gym for most kids. Out of a preteam group of girls who all basically possess good physical qualities for the sport, if you follow them over the years some of the ones who either stall out or quit and some of the ones who rocket ahead or just stay in it slow and steady until they are teenager L10s with good skills would surprise you. It's not always that easy to tell because the factors other than physical attributes and the unknowns like injuries are big factors.
As far as picking out what kids have the potential to make it to team levels, that's pretty natural because it's fairly obvious. There's "class" good and "team" good, I guess that's the way to put it really. There are kids who are physical kids and good all around athletes who can go through a L1 course and do the things you do...but they might never get to L4. And there are kids who get to L4 (or 5, or 6) but won't be optionals. In my entire summer classes, some of the kids are pretty young to tell but I can think of about five kids maybe who I think really have both the physical ability and mental focus where if they want to pursuce gymnastics, they could probably be level 4s in a few years.
Out of the entire session, I can think of ONE girl with the physical ability and ability to translate instructions to movement where I'd say she could be a level 4 in a year if someone really wanted to train her there (in this program it'll probably be two) and a good one - impeccable form for a little beginner kid, unusual coordination, incredible core and upper body strength. Just one. Now some who I say have the basic ability could well keep it up, get through 4, 5, 6 and then suddenly be great optionals and pass her, but her potential for performing basic team skills in the near future is great. She's stronger than a lot of L4 trainers I've seen and has the natural great lines and flexibility that tend to bode well for compulsory scores. Do I get really excited, I like to HER excitement at discovering she is so good at something, just like I like to see every kid enjoy themselves. The talented ones tend to especially enjoy it and feed off your love of gymnastics. But honestly it's not that super exciting because 99% of the time life gets in the way and that kid isn't going to Olympics, L10, and a lot of times not even L4, so being extremely emotionally invested at the expense of others isn't worth it.
