In a healthy 9-month-old no food, including tuna, needs to be delayed to reduce allergies
(in fact, feeding
may reduce allergies).
Great omega-3s in tuna, but watch out for accumulating mercury, especially during first 3 years. Wild salmon is a better first fish. If choose tuna: small, infrequent portions. And 'light' tuna better than 'white'. Some brands have lower mercury (smaller fish) options.