Maggie is immature because she is self-conscious and not sure of herself. She is easily swayed by the interests of others and in her desire not to offend. When she offers to help Tom find a job by learning book-keeping and then teaching him, he recoils in anger. Rather than being annoyed by his bad behavior, she is left feeling guilty. The mishaps that lead her to lose her reputation are a result of not wanting to offend Stephen. She allows herself to be led, and suffers for it.
Maggie gains the conviction and confidence she needs and is able to show it. She finally is able to stand up to Stephen, after his disastrous trap leaves her ruined. She stands by her conviction to honor her brother's wishes, and does not accept Phillip as a husband. She has made the choice to put her brother first and will stick by it. Her coming of age is noticed by others as well; Lucy comments that Maggie is more mature than she is.
Tom, on the other hand, never demonstrates that he has changed. He has a moment of epiphany just before his death, when he admits that he has been unfair and that his priorities were all wrong. This seems like a 'coming of age' moment. However, because he never has a chance to show his new understanding, he can't be the Bildungsroman hero.