"Sin da ragazzo non mangio carne."
Up until now no one said ragazzo can mean childhood. Also "Sin" had no explanation as to what it means in english.
I wonder if it is similar to how in spanish, you can say "desde niño" which is basically the same construction and means "since I was a kid/boy"
That's the case. This construction comes from abbreviating the full sentence.
I understood to talk about that sort of time period, you'd use giovani. Not a very good sentence either really, is it? Needs some other tense to sound correct.
Normally this would be expressed as "I have not eaten meat since I was a boy."
I'd agree with this, the natural construction in English would be "I haven't eaten" rather than "I don't eat" in this case. However, the only tense that has been taught at this point is present tense, though I suppose they could just not include this example.