In the days when weaning started at 4 months, a baby could have tuna after trying the usual fruits and veggies. Mine certainly had it well before 6 months. It is more a texture thing I found as it doesn't puree very well.
Thanks everyone - I'll stick to white fish for now then. It's just that the other ds's eat tuna so I thought it would be easy to take a spoon out for ds3 and mash it with potato or something. I'll keep to cod!
I believe current advice is not to give tuna to pregnant women and infants more than once per week due to possible contamination with mercury. I have no idea how big the risk is - I have been giving tuna in olive oil to ds since he was about 10 months and try only to have it once a week just in case the scare stories have some foundation.
Oh I wouldn't worry Dino. Everything you read gives different advice. I've just read one thing which says no meat or fish at all until 9 months but others who advocate all fish quite early. Heinz has a "from 7 months" recipe with tuna so it can't be that risky.