I think my ds ate raw fish sushi for the first time when he was 4 yrs old. My younger ds had some for the first time at about 2.5 yrs old. You can go out for sushi and not get raw fish (think CA rolls, veggi rolls, etc) but also be careful about it just being really big mouthfulls for a very young child (at least the way my kids eat!), they couldn't really take little bites, though my now 6 yr old generally dissects his sushi completely before eating it in little
bits. You could also do that for your child, just break down the rolls into pieces, a little bite of avocado, a little bite of rice, etc. Enjoy!