Hi Mauricio,
I'm glad to hear that as we get more experience with different parts of the Alice program, it's making more sense to you. I feel the same way - the concept of parameters and variables are a little fuzzy, until you experiment with them in your own animations.
Regarding your question of how the variable got its value, I think of it this way:
when you create a new variable, you pick any value (for Assignment 5, it was a numerical value) as a placeholder.
The value you replace the placeholder with, is created by the 'what is your age' question that you ask the user.
Whatever answer your user gives when they are asked the question, that numerical answer then becomes the value in the variable.
Hope the above makes sense! Stick with it, we're all in this together and the concepts will start making sense the more practice we get.