Can a 9 month old infant eat tuna?

Yes, but... 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.
In small amounts. Because of the mercury found in nature, that big fish accumulate along their life span, you should limit tuna intake in pregnant women and children to recommended allowances per month. Less than 40 g per month for infants.