It seems to me that Constitutional rights would apply to all adults, which is considered to be the age of 18.
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Children's Rights
children's rights: an overview
A child is a person and not a subperson over whom the parent has an absolute possessory interest. The term "child" does not necessarily mean minor but can include adult children as well as adult nondependent children. Children are generally afforded the basic rights embodied by the Constitution. See Civil Rights of Children. The Equal Protection Clause of the 14th Amendment is said to apply to children, born within a marriage or not, but excludes children not yet born. There are both state and federal sources of child-rights law.
https://www.law.cornell.edu/wex/childrens_rights