At what age can a child get an anti-tounge thrust appliance?

Debatable: Such appliances are very uncomfortable, some have spikes and are painful to kids. They interfere with eating and sometimes language. Why do you not try a speech pathologist, many have experience with tongue thrusting.

6 years (usually): Most tongue thrust appliances hook onto the 6-yr molars, so this is about the earliest one could be done. There are different options here, so speak with either an orthodontist (recommended) or your dentist to discuss the best way to approach this.

Tongue thrust: Most tongue thrust due to narrow upper jaw. Solution is to widen jaw, not constrict tongue. Speech therapy significantly more effective if environment normalized first. See Orthodontist who will team with Speech Pathologist for most effective treatment. As to when, malocclusions are like snowflakes, no two are identical. See Orthodontist NOW to plan type and timing of treatment.