When can parents let go their kids?

I can't imagine what you are going through. It won't be long until I have to let my oldest one go and I don't think I am ready. I know I have done a good job with him but still don't know if he can make it in this big world by himself. I think they are alway children to us.

Depends on when the child has matured enough to take care of itself .

Before she turns eighteen, you'd better keep an eye at her. If she is not over eighteen, she can't be responsible for their actions. But after she turns eighteen you should let her go. Not letting her go like: "Leave and never come home". Letting go means you'll let her know what is it like to be free, what is life like. She must still visit you often, don't forget about that! If she doesn't, then you should do.