There is no answer. Physically the eyes can tolerate contact lenses at a very young age.
The question is more "is the child mature enough to wear contact lenses?" Children are naturally great contact lens wearers if they accept the responsibility for them.
Parents must look at how the child handles other responsibilities, like does the child take out the trash, care for the dog or follow through with school or other projects? If the child needs frequent reminders for everyday chores, the child may not be ready for the responsibility of wearing and caring for contact lenses.
Talk with your eyecare practitioner as well if you feel your child is not handling the switch from glasses to contactlenses well. Contact lenses can always be tried again at a later date.