Most animals establish their life-long diet when they are rather young, though it may change over time.
For example, many water turtles start out as mostly carnivores, and as they get older, the switch to a largely vegetarian diet. So, you need to consider that, too, when feeding your animal.
Wild turtles eat a variety of foods and are quite opportunistic.
Captive turtles are often raised on one and only one food, and it can be challenging to get them to try something new, especially if the food they are used to is also one they really, really like.