The law allows an 18 year old to freely spend an inheritance. If you feel this is unwise, your trust can restrict when and how the inheritance will be used. For instance, your Trust can state that the inheritance will be used only for necessities, such as shelter, food, education, etc. After that, the inheritance will be given to the child in increments (i.e., one-third at age 25, one-third at age 30 and the balance at 35). Or you can give the trustee discretion in deciding when and how much to give the child. You are limited only in your imagination in writing conditions into the trust. You should also name a guardian for minor children.