How can I talk to my parents about my vegetarianism?

Look, I am attempting to transition into vegetarianism, and there aren’t too many obstacles getting in the way of going to that path (okay, I still want bacon on occasion, but there are alternatives), except for my parents. They are concerned that:

A: It will “stunt” my growth (which is to them is due to a “lack of calcium”, which I believe to be getting in the same amount as pre-vegetarian days)

and

B: Something about poultry and fish (yes, forced pesce-pollotarianism).

How do I discuss with them that I truly want to become a vegetarian and their arguments are invalid?

Sidenote: I get most of my calcium via soymilk, which I have always preferred to dairymilk and cheese, which I am not so sure about eliminating. I do eat B12 fortified cereal.