I wanted to know the age limit to do a biofeedback scan and treat. I have child under age of 3 who has eczema and want to scan and treat.

From Nenah Sylver's Rife Handbook p325:

"German naturopath Harald Sievert writes: “In my opinion, children are far more resilient than we adults believe. This is clearly demonstrated in our therapeutic measures where children eliminate better, react quicker and produce positive results more rapidly with holistic treatment.” 

This suggests that a child will be strong enough to handle the microbial die-off from a session.
Rifing infants should be done on a case-by-case basis, as infants are fragile. The brain and nervous system, digestive tract, reproductive organs, endocrine system, and immune function are immature and still developing.

Nonetheless, should your children ever be ill enough to warrant a doctor’s prescription for antibiotics, this may be the time to rife them instead. Unlike drugs, rifing does not contain chemicals that poison the body. Many parents have reported success with infants even as young as a few months old.

If my infant or small child had a dangerous disease for which a doctor wanted to administer toxic drugs, I would first try rifing without hesitation. However, it may be unwise to use the technology for routine and relatively mild childhood illnesses (such as the German measles) unless the child is excessively uncomfortable or ill. These illnesses stimulate the child’s system to produce antibodies (proteins that catalyze the body to defend itself against foreign microbes)."

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https://www.spooky2.com/forums/viewtopic.php?f=10&t=9251