 Being people of honest integrity requires us to know our own limitations, and isn't that exactly what Jesus suggests here? You cannot make one of your hairs white or black, he says in verse 36. You might make all sorts of promises, but God decides whether or not your hair turns white or black, which is just another way of saying God decides how old you're going to be when you die. God decides whether or not you're going to grow old, and so we should say, if the Lord wills.