What you refer to is known as the Rolling Mating System; it is a very old traditional farm method of breeding: cocks with pullets; stags with hens. There are several variations on this system; including the use of mature hens and cocks only.I remember back in the legal days several of the top cockers found that using pullets produced their best Warriors ...... they were using the pullets with mature cocks . Also heard this done in reverse .
I've also heard that young chickens are at their best reproductive stage (young hens/pullets and bull stag) they will posess in their lifetime .
Nothing wrong with waiting for maturity especially if one has plenty of fowl and resourses ...... being careful never hurts one at all .
Just remember there are Exceptions in everything ....... just hang around long enough and you will see it .
Among the very best Oriental breeders, in the States and in their homelands, any system which utilized such young fowl was never used or recommended.
Traditionally among Asil breeders cocks were never used before they were 4 years of age with many waiting until the cocks were 6 years old.