I don’t think there is an “official” definition of tender age in terms of what exact age it refers to. Most main dictionaries define tender age as:
a time in your life when you are still young and lack experience.
It appears that tender age limits are defined according to context, for instance during recent episodes of detention centers along the U.S. southern border with Mexico, the following article from ABC says:
What classifies a child as 'tender age'?
Border Patrol officials said children under 5 fall under the definition of "tender age."
Border Patrol chiefs in the field have generally referred to children "of less than 5 being tender age," Hastings said.
But according to an official from the Department of Health and Human Services, a "tender age" child can be up to 13 years old.
"We have specialized facilities that are devoted to providing care for children with special needs and tender age children as we define as under 13 would fall into that category," Steve Wagner, HHS Acting Assistant Secretary, Administration for Children and Families told reporters Tuesday.