Varies. Some dentist will do mixed dentition braces, before the baby teeth have been lost. Some will wait until the adult teeth are in. It depends on the severity of the underlying problem is, and the steps needed for correction. Discuss with your child's dentist.
Depends. The need for rx is based on individualized diagnosis, no 2 children are exactly alike. Orthodontists recommend 1st ortho cm-up age 7, because sometimes we can do early rx to reduce complexity of later full ortho rx. For most girls, braces at about 11, it's at about 12 for boys. Rx should start just before loss of the last baby teeth and coincide with pubertal growth. See qualified orthodontist.
Depends on problems. The best age for orthodontics is very dependent upon the specific needs of each patient. Some children may benefit from some limited early care before all permanent teeth erupt. The majority begin treatment as adolescence begins, start of teen years, and the growth spurt begins. It's best to ask your orthodontic specialist what is best for your child.