Can an adult child collect back child support if they were emancipated at the age of 16? What about if they got married at 16?

no. Child support goes to the custodial parent. Its not the child's to collect. Emancipation ends it.

No, chiild support is not owed ever to the "child" it is owed to the guardian or parent of that child for the expense of raising them.

At they point they are emancipated, normally it ends all current support. The back support is still owed to the parent who it is owed to