I don’t subscribe to this theory. I think you have to take physical maturation into consideration. I prefer that kids do multi-events. I would prefer kids be taught stuff like hurdles and obstacle negotiation at young ages, 7-12 years of age, in physical education. In fact I want kids to be multi-sport capable for as long as possible. I have made the following observations in my life, physical maturation and when it is completed is the largest governor in development of speed. If you mature early and aren’t running 10.8s for 100m then you are looking at a 10 year span to possibly get to 10.5s range if you are a 10.9s 100m runner. However, a kid running 11.4s for 100m who hasn’t physically developed by any age has potential for sub 10.5s range.
Also, I still wouldn’t take a 51s guy and make him a 1600m runner, he’s a 400/800 runner and not a miler at worst. A 51s 400m automatically is a potential 1:52 guy at 800m. There is also a little caveat I have seen with 51s 400m runners since there are so many of them. They are being trained wrong to begin with limiting 400m potential. Personally, I am a better miler and 100m runner than I am a 400m runner or 800m runner with training that resembles most HS track and field training. If you profiled the HS coaches who use Clyde Harts 400m training or something similar you would find almost all don’t alter his prescription for 46-47s 400m runners when he gets them for their HS athletes who come in out of Jr. High running 53-56s for 400m. So why is these same runners max out at 49-51s for 400m regardless of maturation, but others can get their kids under 49s by not adhering to large training volumes. I try to see how my athletes respond to speed and lifting in training. I like to see how they learn by experimentation with hurdles, high jump, and discus. I use those as my movement screens to understand the abilities my athletes have. When I couple those technically demanding events with speed/power events like the 100m, Long Jump, and Shot Put I have an idea of what events fit them and have introduced them to other events. Most of my athletes don’t like the 600m or 1K time trials i put them through. When I see time and space awareness coupled with high speed and power numbers and decent endurance. I know I have a multi-eventer. When I see good speed and power with good endurance, I have a mid which can do sprints or jumps, When I see time and space awareness along with good endurance I have a long hurdler, maybe a decent discus thrower and high jumper. When I see an athlete is only good at one of these areas, I know have work to do.
I think you have to balance steering them and them having fun. If they aren’t having fun then its likely they won’t perform and when they don’t perform well they will likely quit.