Fluffies are very similar to human children (logically enough, since Hasbio secretly used human DNA in their construction), the main difference being that fluffies have more extensive instincts (programming) than humans.
So if a fluffy is treated as a human child the way Billy clearly was, it makes sense that they would develop into the fluffy version of a mature adult.
Most domestic fluffies are treated like pets, their owners don’t really teach them anything beyond the basics, so they never reach ‘adulthood’, barring the occasional mutant with unusual mental capabilities.
Feral fluffies have a different path to ‘adulthood’, the rare fluffy that survives ‘the school of hard knocks’ may develop a certain maturity.
Obviously this is my headcanon, but its based on a wide variety of stories from the Booru and I think its broadly applicable.
God I love the fluffy concept, its great science fiction.