Why were Romeo and Juliet too young to understand what true love really is?
Of course, this is just an opinion. You cannot say for sure that they really were too young. After all, it is pretty hard to define what exactly true love is. It is also hard to say when, exactly, someone goes from being too young to being old enough.
To me, there are only two ways to argue this. First, you can simply point out how young these people are. We don't know how old Romeo is, but we know Juliet is only 13. That is awfully young to be falling in love and getting married.
Second, you can say that they prove that they are too young by their actions. You can say that they just rush headlong into everything without thinking about their actions. They get too caught up in their emotions rather than taking a step back to see if there might be other options.
I would have to argue that we really do not know for sure if they were in love or not. Some of their actions were indeed quite childish but this does not prove or disprove whether they were in love or not.
Also important to this question is their young ages. To us they are certainly too young to fall in love and get married. During their time however, young marriages were very common. Life expectancies were not as high as they presently are. People got married earlier in life.
On the hand, maybe their actions proved how very much in love they actually were. Perhaps they knew that they could not live without each and that's why they took the actions that they did.