both male and female cats can start breeding as young as four months old. as far as personality...cats are like humans there. some cats mature into quiet, settled animals while others never outgrow their kittenish love of playing. i have a nearly ten year old cat that loves to play...with the other cats or by himself with toys. if you are wanting him to grow up faster because he keeps you awake at night, consider getting him a playmate. also cats all grow at different rates. your cat may have a breed in him that is large, which would make him bigger than other cats his age, while it may also be true that he was just born to be larger. all kitties are different. see if you can find another 5 month old kitten you can compare his size to to see if it is just your perspective or he really is large for his age.