What is a good age for orthodontic treatment for a child with underbite?

As soon as possible. Affects child ability for speech quality, certain alphabetic letters become difficult to pronounce, modulate. Depends on the defority reason/etiology, intervention & procedure may be simple, rare cases may need surgical intervention at late follow up.
Depends. All children should be seen for 1st ortho check-up @ age 7, especially if upper jaw too large. If problem is dental rather than skeletal, rx may be needed about the start of puberty. If problem skeletal rather than dental rx may need to be put off until all growth has ceased. See a qualified orthodontic specialist to evaluate your child's individual needs. Don't put it off, go now.
Depends. A true skeletal underbite (large lower jaw) needs to be monitored, but cannot be fully corrected until the lower jaw stops growing. If the underbite is due to a smaller than normal upper jaw, reverse pull face mask therapy can begin prior to the age of 10 to help decrease the discrepancy. Again, the etiology of the underbite is most important to know prior to beginning treatment.
Depends. Usually a severe underbite is best treated after the child has completed all growth. The lower jaw tends to grow more and for a longer time than the upper jaw, so if treatment is completed at too early an age, the patient can outgrow the treatment and re-develop an underbite.
Usually 8. One must understand that there are many types of underbites; some of dental origin, some of skeletal. If it is dental or very mild skeletal then early intervention could be very beneficial, and as an orthodontist I have more control with so much growth remaining. If it is genetic and severe, then early orthodontics will not be of much help, and must consider surgery as an adult.
Earlier is better. We try to treat underbites by age 6-7. Left untreated, facial growth is influenced negatively.
7 years old. The american academy of orthodontists reccommends that children be evaluated at age seven in general. Some kids can wait longer, but in the case of an underbite, you really need to have them evaluated at 7yrs. This is one of the dental development issues that is most important to begin treatment on early!