At what age does a child get ready for a smart phone?

Once you feeling they are mature enough and they understand the dangers that could arise and how to avoid such situations. I would like to be assured by my child that they would approach me if things went wrong or they needed support that way I know they trust me enough to keep me involved with things that are going on in their lives. I would want them to know that having a phone is ok but breaking boundaries and misusing is not ok!