Contrary to popular belief, people suffering from selective mutism do not necessarily improve with age, or just grow out of it. Consequently, treatment at an early age is important. If not addressed, selective mutism tends to be self-reinforcing, with the person being known as the one who doesn't speak which makes it all the harder to then speak. Sometimes in this situation, a change of environment (such as changing schools) to a place where the condition is not known can make the difference.
Occasionally, treatment in teenage years becomes more difficult, though not necessarily.
Forceful attempts to make the child talk are not productive, usually resulting in higher anxiety levels and so reinforcing the condition. The behavior is often viewed externally as willful, or controlling, as the child usually shuts down all communication and body language in such situations, which is perceived as rudeness.
The exact treatment depends a lot on the subject, their age and other factors. Typically stimulus fading is used with younger children.
Some in the psychiatric community believe that anxiety medication may be effective in extremely low dosages and that higher doses may just make the problem worse. Others in the field believe that the side-effects of psychiatric medications- in any dose and on any child- are so dangerous as to negate any temporary benefit, preferring behavioral and psychological interventions.