Do you mean, "below what age does the child NEED a baby monitor on, for safety reasons" or "at what age should a child's need/desire for privacy override the parent's interest in spying on the kid"?

My answers, zero, and maybe 3 or 4. I don't think they're necessary for safety for a normal kid; if the kid has some kind of medical condition that may be necessary. I think as soon as the kid has anything that the parents could be spying on the kid about, they should probably give him space. I guess I can't care much about parents using technology to spy on their kids if the kids don't care either.