"You are a boy, I am a man."
Translation:Tu sei un ragazzo, io sono un uomo.
Il, lo, la and their elided form l' are definite articles, equivalent to the English 'the', while un, uno, una and its elided form un' are indefinite articles, equivalent to the English 'a' and 'an'. To pick the right article you also need to know the gender of the noun it refers to (il, lo, un, uno are masculine; la, una, un' are feminine; l' can be both) and the letters following the article (too long to write here).
Yes, Lei substitutes tu as a polite and formal address, as most Romance languages do with the 2nd person plural (French vous, Spanish usted, Italian can use voi with this meaning as well, but with a lower degree of politeness like in the Portuguese você) and German does with the 3rd person plural (Sie); duolingo and many institutions recommend capitalizing in this case to distinguish it from the 3rd person lei.
I am italian ,and usually i think that Bambino/a is used up to 10 or maximum 11 years old, the age that is related to the childhood, the end of primary school in italy (11),. i have always considered the teens starting at 12 years old,. and in that moment usually i use "Boy" instead of "Child"..but it is my usage.Though, is more or less like this.