I think Maine coons take about 4 years to mature fully..my guy Montanna is 11, and he changes all the time.. his fur is quite short arround his head, but in winter he gets his mane.. and it's quite long, but in summer he looses it.. but his fur is changing colour now.. he is a brown tabby colour, now his under coat is turning a golden orange and looks super pretty.. but he seems to be getting smarter all the time..He watches his two younger siblings and before I know it he's doing what they do, who says you can't teach an old cat new tricks! I also read that cats fur acts as insulation in summer, so maybe that's why Charlie's coat is getting longer to insulate him from the heat..