Human beings are about the slowest physical developers of all mammals ... and with good reason ...

... because being a successful human is as much a matter of mental ability as physical ability ... and the slow physical maturation process allows time for the complex mental development to take place.

As far as a 2YO horse being the physical equivalent of 6YO or 8YO humans ... utter nonsense. There are 2YO horses who are as large as 17 hands ... the equivalent of a 6'6"+ human ... and when was the last time you saw a 6YO kid who could slam dunk?

Sheeesh !!!