Matoran can die of old age. In BIONICLE Legends 10, a group of Av-Matoran died of old age and turned into Bohrok. Takua just happens to live longer than the others.And even though Takua/Takanuva is the oldest Matoran alive, it really doesn't matter anyway. Not everyone in the story knows of that fact. Not even Takanuva himself knows that he's the first Matoran. So it really doesn't have much of an impact on his personality. Well, not yet that is.