There is no definitive answer to this question as it depends on a number of factors, including the type of knife, the kitchen environment and the level of supervision. However, as a general rule of thumb, most experts recommend that children be at least five years old before using any type of knife in the kitchen.
Of course, even at this age, it is important to closely supervise young children when they are using knives. For instance, you may want to consider having them use a plastic or wooden knife rather than a metal one. And always make sure they understand the proper way to hold and use a knife before allowing them to use it unsupervised.
As your child gets older, you can start giving them more responsibility in the kitchen. For example, they may be able to help prep meals by chopping vegetables or slicing fruit under your supervision. Just remember to go slowly and always stress safety first!