It should be "I've been doing / I have done this job for 13 years." There's no real difference except maybe the second is more factual and the first emphasises continuity, although tone of voice could alter both..
To explain: 'since' must relate to a moment in time, not to a duration of time. For example "since I joined the company 13 years ago" refers to a moment.
Duo's Italian refers to a duration. In English you say simply "for" a duration, not "since". Duo's error is a classic symptom of a non-native English speaker, and is very common. Don't do it.
It does NOT mean "since I was 13 years old"; that would be da quando avevo 13 anni.
This is not correct English, and the "correct answer" shown above is different than the one given in the exercise: "I have done this work for 13 years", which would be correct / natural - and what I would have written, except the example used "faccio", which is present tense; "I have done" would be "ho fatto"