I'm thinking of letting my 20 month old son handle the knife.
Why would you let a youngster play with a knife?
The Sun has a different opinion. It says a child can manage a knife at 2 years old
Letting a toddler handle a weapon is dangerous. They lack the ability to make the appropriate decisions necessary when handling a weapon. They could injure themselves or others with that knife.
Maybe, I think it's better to let him practice it.
Maybe use a butter knife before giving him a real knife. It is a lot safer that way and I will feel better about him playing with it.