A minor may validly waive his or her Miranda rights. The test is whether, under all the circumstances, including age, intelligence, and education, the minor had the ability to comprehend the meaning and effect of a confession. People v Lara (1967) 67 C2d 365, 385, 62 CR 586; In re John S. (1988) 199 CA3d 441, 445, 245 CR 17.
My standard disclaimer: I am not offering legal advice, assume I do
not know the law in your state or at all for that matter and that I am
just making suggestions for starting points for when you do speak with
an attorney. Do NOT rely on anything I write and contact a lawyer in
your area immediately after reading my posting.