Depends on the toddler's character and understanding level.

I was using Swiss army knives as early as 5-6 years old. I probably cut myself thrice at most.

However, if the toddler is the reckless kind, they'll start with a plastic butter knife.

I have cut myself once, and only once with a sharp knife. I was about 7 years old, and my grandfather gave me a new (sharp!) Swiss army knife.