Why are some humans more mature than others? Yes it has a huge amount to do with how they are handled from a young age but horses have different characters and therefore are all different.
Do you mean physically or mentally mature? I think mentally mature is all due to handling from a young age. My 3yr old is a little saint and better behaved than my 19 yr old broodmare!! Whereas physically he is still very weak and not well covered compared to other 3yr olds ive seen (despite having hay at night and excellent grazing during the day). Im not sure what this is due to, maybe because he was my mares first foal, or maybe hes just going to be slow to develop physically, he is also going to be a big boy so this could also have something to do with it.
I personally think alot has to do with the Breed aswell. I have a 2 year old WBxTB and am still waiting to see if he takes the TB quick development or the WB late development!