Good question. I don't think the characters mature at all. When Romeo kills Tybalt, he yells that he is "fortune's fool". To me, this indicates that he has not taken personal responsibilty for his actions. When he is in Friar Laurence's cell, the Nurse has to tell him to "Stand up, stand up; stand, an you be a man/ For Juliet's sake, for her sake, rise and stand" as he is on the floor bawling his eyes out.
As for Juliet, I say that she has not matured through out the play because her character has been constantly mature. She is the thinker out of the two of them. She recogonizes the results of her decisions. In her last soliloquy in act four scene three, she weighs out the pros and cons of drinking the potion. She could not wake up. She could wake up and still have to marry Paris. She could be trapped in the Capulet tomb forever. This is quite a bit different from Romeo and his rash decision making skills.