As someone who is 18 and already an adult, it is perfectly moral for you to enter into full Communion with the Church, without discussing it with your parents.
That said; as Catholics, we're called to share our faith.
Your situation is a bit unique. You may already, canonically, be a Catholic in full Communion with the Church as you describe your situation; I'm not sure.
Your mother is Catholic. You were raised Catholic. The only difference is you were baptized and confirmed in the Armenian Apostolic Church, which has valid Sacraments so you need to discuss your status with a Catholic Priest.
At a theological level, the term converting, or conversion isn't really accurate or used anymore for Orthodox or Oriental Christians, because all your sacraments are valid. We simply say entering full Communion but, in your case, you were brought up in full Communion as a Catholic.
Returning to your parents: I suspect that their objections might be more about the unique ethnic and cultural traditions of the Armenian Apostolic Church.
I live in an area where we have many Armenians and (2) two Armenian Apostolic parishes so I am aware of the strong bond between the Armenian Apostolic Church and the Armenian people but it's also the case with the Armenian Catholic Church.
As you pointed out, your mother is Catholic. She brought you up Catholic.