Why don't you and Julián eat? Why not have something to eat?
How old am I? You have come nine years.
In the first sentence, Mario's answer is using the auxiliary verb "have" to suggest that they should eat something. The phrase "have something to eat" is a common way to express this idea.
In the second sentence, Mario's answer is using the verb "come" in the sense of "reach" or "attain". So, he is saying that his little sister has reached the age of nine years old. This is a colloquial way of answering the question about age, rather than saying "you are nine years old".