Can a child, subject of a custody who is now 18 file to intervene in the case?

This would be highly unusual, and very doubtful. The court has authority to address custody of the child only until the child is 18 years old. So, a child seeking to intervene in a case when he turns eighteen would be doing so when his custody is no longer an issue, because the court has no further authority over his placement and living arrangements. Child support can remain an issue until the child is nineteen, but that is an issue of payments between the parents.