From what I've read, the "rules" are slightly more complicated.
http://www.french-linguistics.co.uk/grammar/tu_and_vous.shtml
According to that, not only can you say "tu" to twister because of your age range, you being a teenager and she being in her twenties, but as participants in an online forum it's common practice to just use "tu", whatever the age gap may be.
Courtesy forms are basically the same in italian, with one big difference:
the "tu/Voi" was in use until the 16th century, when "Voi" was gradually replaced by "Lei", which is the feminine pronoun, in its accusative form.
Wiki sez:
Lei was originally an object form of ella, which in turn referred to an honorific of the feminine gender such as la magnificenza tua / vostra ("Your Magnificence") or Vossignoria ("Your Lordship").[11] Example: "Gino, Lei è un bravo ingegnere. Marco, Lei è un bravo architetto. Insieme, voi sarete una gran bella squadra" "Gino, you are a good engineer. Marco, you are a good architect. Together, you will be a great good team".
more on that here
http://www.pimsleurapproach.com/resources/italian/grammar-guides/formal-informal/
and here
http://italian.about.com/library/weekly/aa081899a.htm
The "Voi" form was briefly in use during the fascism, and still remains in use in small pockets in the southern regions, in some dialects.