Romeo handles his rejection of the love from Rosaline by isolating himself, being depressed, and very emotional. Romeo has felt like he completely lost himself over not being loved back by Rosaline. In his conversation with Benvolio, "...I have lost myself; I am not here:
This is not Romeo,he's some other where.
By being extremely depressed over a girl not loving him back, shows Romeo's adolescence and inexperience because someone older who has experienced rejection before, would most likely be able to handle the situation instead of acting like it was the end of the world.
Romeo also shows his youth when he notices that Juliet's lips are stil warm, that there is still color in her body, but instead of thinking of what could have happened other than death, he is very quick to make such an extreme deciscion such as suicide.
If he had been older, he may been able to understand that maybe Juliet was indeed NOT dead, which would have saved the couple completely from death.
In Act V, when Juliet realized she would no longer live life with Romeo, she killed her self right away. Adults lose loved ones often, and although yes, Romeo's death was tragic, if Juliet was to understand that she could live without him because people live without the person they love every day, she would not have killed herself.